We have developed a multichannel output coupler enabling coherent beam combining in the two-micron spectral range. We demonstrate experimentally the combining of multiple beams utilizing a set of thulium-doped, double-clad, singlemode optical fibers. The fibers are pumped by fiber-pigtailed laser diodes at 793 nm via (4+1)x1 pump-signal combiners. The combiners are fabricated using vanishing-core technology, which allows for preservation of the mode field through the tapering process. The output of individual lasing channels is generated over a 20 nm spectral band at around 1970 nm without any spectrally selective elements. The slope efficiency of individual lasers is approximately 50% with respect to the pump power. All lasing channels are fused into a monolithic silica structure with channel spacing of 32 microns on a triangular lattice. The fused assembly is fabricated in a glass microforming tapering process with a draw ratio of 3.9. In the process, the mode field at 1970 nm expands slightly to about 15 microns at the end of the taper, while the outer diameter is reduced from 2.3 mm to approximately 590 microns. The tapered end is straight polished and fusion spliced to a 600- micron diameter silica glass rod. The rod is cleaved and optically polished at zero degrees. The length of the rod is one half of the Talbot distance for optimal coherent beam combining. In the experiment, an antiphase supermode is observed when only the seven inner channels are pumped, and an in-phase supermode is excited when the number of channels is nineteen or larger.
In response to the optical packaging needs of a rapidly growing silicon photonics market, Chiral Photonics, Inc. (CPI)
has developed a new generation of ultra-dense-channel, bi-directional, all-optical, input/output (I/O) couplers that bridge
the data transport gap between standard optical fibers and photonic integrated circuits. These couplers, called Pitch
Reducing Optical Fiber Arrays (PROFAs), provide a means to simultaneously match both the mode field and channel
spacing (i.e. pitch) between an optical fiber array and a photonic integrated circuit (PIC). Both primary methods for
optically interfacing with PICs, via vertical grating couplers (VGCs) and edge couplers, can be addressed with PROFAs.
PROFAs bring the signal-carrying cores, either multimode or singlemode, of many optical fibers into close proximity
within an all-glass device that can provide low loss coupling to on-chip components, including waveguides, gratings,
detectors and emitters. Two-dimensional (2D) PROFAs offer more than an order of magnitude enhancement in channel
density compared to conventional one-dimensional (1D) fiber arrays. PROFAs can also be used with low vertical profile
solutions that simplify optoelectronic packaging while reducing PIC I/O real estate usage requirements.
PROFA technology is based on a scalable production process for microforming glass preform assemblies as they are
pulled through a small oven. An innovative fiber design, called the “vanishing core,” enables tailoring the mode field
along the length of the PROFA to meet the coupling needs of disparate waveguide technologies, such as fiber and onchip.
Examples of single- and multi-channel couplers fabricated using this technology will be presented.
We have developed an all-glass, fusion spliceable polarization maintaining (6+1)× 1 pump/signal combiner for fiber lasers and amplifiers. We utilize an enhanced tapered fiber bundle technology for multimode pump channels and a vanishing core fiber for the single mode polarization maintaining large mode area (PLMA) signal channel. The signal channel of the combiner is optimized to match a double-clad PLMA fiber with 20 micron core and 400 micron glass cladding with 0.065 numerical aperture (NA). The multimode pump channels have 200 micron core and 240 micron cladding with NA of 0.22 designed to deliver high power 980 nm pump light. The same double-clad PLMA fiber is used as both the signal input channel and the combined output for the device. Polarization axes of the input and output PLMA fibers are aligned during the fusion splices to achieve polarization crosstalk below -20 dB. Utilizing this approach, we have achieved coupling loss of ~0.4 dB for the signal channel as measured from the input PLMA to the output PLMA at a wavelength of 1060 nm and coupling loss below 0.01 dB for all pump channels as determined from the measured temperature rise of the combiner package temperature as the optical pump power at 974 nm is increased up to 45 W. Low signal and pump losses result in high efficiency lasing or amplification at over a kW of pump power for high power applications where a single mode, high polarization extinction ratio output is required.
KEYWORDS: Optical fibers, Signal attenuation, Waveguides, Phase modulation, Fiber lasers, Polarization, High power fiber lasers, Optical arrays, High power fiber amplifiers, Optical amplifiers
We have developed a (6+1)x1 combiner for fiber lasers and amplifiers based on a glass fusion technology. We have
combined a conventional fiber fusion technology for pump channels with a new design for a single mode signal
channel, which utilizes a vanishing core technology. The approach has been developed for single channel spot size
converters and pitch reducing optical fiber arrays (PROFAs). Flexibility of this technology allows a custom design
to match both a single or large mode area fiber at the input and a required active fiber at the output. The technology
allows two parameters, mode field diameter (MFD) and taper diameter or channel spacing to be adjusted independently resulting in low loss coupling for signal channel at input and output. Utilizing this approach we have
obtained better than 0.3 dB coupling for a signal channel at 1550 nm with a standard SMF28 fiber at the input and
an active fiber at the output, while using six conventional 105/125 micron fibers as pump channels operating at 974
nm efficiently coupled to a double-clad fiber. Low signal loss results in high efficiency lasing or amplification suitable for high power applications. This unique technology allows excellent coupling for the signal channel as well as for the pump channels and is amenable to even more pump channels if desired.
We have fabricated both pressure and temperature sensors based on chiral fiber gratings that can operate in harsh
environments over wider measurement ranges than conventional fiber Bragg gratings (FBGs). Chiral fiber sensors
are made by twisting one or more standard or custom optical fiber with a noncircular or non-concentric core as they
pass though a miniature heat zone. Because the resulting structures are as stable as the glass material, they can
operate in harsh environments. Excellent temperature stability up to 900°C is found in pure silica chiral fiber
temperature sensors. We developed a correlation algorithm for use with a standard FBG interrogator to accurately
measure the shift in the transmission spectrum as the environment of the sensor changes. We developed a calibration
procedure, which allows the chiral temperature sensor to operate at temperatures from 200 to 900°C with a
maximum difference in temperature reading from a calibrated thermocouple of +/- 2°C. We have fabricated a
transducerless pressure sensor (i.e. no moving parts) operating from 1 atm. (14.7 psi) up to 12 kpsi with a resolution
of 1 psi that can operate at temperatures as high as 700°C.
We have fabricated chiral fiber long-period gratings (CLPGs) for radiation sensing by co-twisting two standard
optical fibers or twisting a single custom optical fibers with nonconcentric core as the fibers pass though a miniature
oven. The wavelength shift of transmission dips in the CLPGs have proven to be much more sensitive to ionizing
radiation than are fiber Bragg gratings. The radiation sensitivity of these CLPGs was investigated in a wide variety
of twisted fibers at the Fraunhofer Institute for Technological Trend Analysis by Henschel et al.7. Because chiral
fiber gratings do not rely on glass photosensitivity, as is the case for fiber Bragg gratings (FBGs), chiral radiation
sensors can be fabricated from a range of glass combinations selected strictly for their sensitivities to radiation in
different circumstances. The fiber may also be made of glass selected to be radiation insensitive so that the fiber can
be used to sense temperature in high-radiation environments. Radiation-induced shifts of up to 10 nm are observed
in transmission dips of CLPGs for doses of 100 kGy of Co-60 gamma radiation. With such high sensitivity, these
gratings can be used as radiation sensors for doses below 10 Gy. The wavelength shift was found to depend upon the
radiation dose rate. This dependence is found to vary with glass composition. This opens up the possibility of using
two CLPGs to simultaneously measure both the dose and rate of radiation.
We have fabricated a variety of chiral fiber sensors by twisting one or more standard or custom optical fibers with
noncircular or nonconcentric core as they pass though a miniature oven. The resulting structures are as stable as the
glass material and can be produced with helical pitch ranging from microns to hundreds of microns. The polarization
selectivity of the chiral gratings is determined by the geometry of the fiber cross section. Single helix structures are
polarization insensitive, while double helix gratings interact only with a single optical polarization component. Both
single and double helix gratings may function as a fiber long period grating, coupling core and cladding modes or as
a diffraction grating scattering light from the fiber core out of the fiber. The resulting dips in the transmission
spectrum are sensitive to fiber elongation, twist and temperature, and (in the case of the long period gratings) to the
refractive index of the surrounding medium. The suitability of chiral gratings for sensing temperature, elongation,
twist and liquid levels will be discussed. Gratings made of radiation sensitive glass can be used to measure the
cumulative radiation dose, while gratings made of radiation-hardened glass are suitable for stable sensing of the
environment in nuclear power plants. Excellent temperature stability up to 900°C is found in pure silica chiral
diffraction grating sensors.
We propose an in-fiber chiral optical isolator based on chiral fiber polarizer technology and calculate its
performance by incorporating the magnetic field into the scattering matrix. The design will be implemented in a
special preform, which is passed through a miniature heat zone as it is drawn and twisted. The birefringence of the
fiber is controlled by adjusted the diameter of a dual-core optical fiber. By adjusting the twist, the fiber can convert
linear to circular polarization and reject one component of circular polarization. In the novel central portion of the
isolator, the fiber diameter is large. The effective birefringence of the circular central core with high Verdet constant
embedded in an outer core of slightly smaller index of refraction is small. The central potion is a non-reciprocal
polarization converter which passes forward traveling left circularly polarized (LCP) light as LCP, while converting
backward propagating LCP to right circularly polarized (RCP) light. Both polarizations of light traveling backwards
are scattered out of the isolator. Since it is an all-glass structure, we anticipate that the isolator will be able to handle
several watts of power and will be environmentally robust.
The "vanishing-core" tapered coupler is an all-fiber device that efficiently couples light between a standard low-numerical-
aperture (NA) waveguide, such as a standard silica fiber, and a high-NA waveguide or device with a
dissimilar mode field profile, such as a planar waveguide or laser diode. The coupler is comprised of a central core
surrounded by a concentric secondary core for low-NA coupling on one side of the device. The central core effectively
disappears on the tapered end of the device. Light escaping the "vanishing-core" at the tapered end of the fiber is
confined in the secondary core by the surrounding cladding for high-NA coupling. This lens-less, low-insertion-loss
solution obviates the need for a significant air gap between coupled components required in lensed fibers and thereby
enables the use of index matching compounds. The all-fiber design also facilitates polarization selective and polarizing
coupling and provides a path towards a high density of passively aligned interconnects.
Chiral fiber gratings are produced in a microforming process in which optical fibers with noncircular or nonconcentric
cores are twisted as they pass though a miniature oven. Periodic glass structures as stable as the glass material itself are
produced with helical pitch that ranges from under a micron to hundreds of microns. The geometry of the fiber cross
section determines the symmetry of the resulting structure which in turn determines its polarization selectivity. Single
helix structures are polarization insensitive while double helix gratings interact only with a single optical polarization.
Both single and double helix gratings may act as a fiber long period grating, coupling the core and cladding modes. The
coupling is manifested in a series of narrow dips in the transmission spectrum. The dip position is sensitive to fiber
elongation, twist and temperature, and to the refractive index of the surrounding medium. The suitability of chiral
gratings for sensing pressure, temperature and liquid levels is investigated. Polarization insensitive single helix silica
glass gratings display excellent stability up to temperatures of 600°C, while a pressure sensor with dynamic range of
nearly 40 dB is demonstrated in polarization selective double helix gratings.
We describe the development of fiber chiral gratings and discuss salient similarities and differences from planar chiral structures. Planar chiral structures include cholesteric liquid crystals and structured thin films produced by oblique deposition of dielectric materials on a rotating substrate. These are composed of uniform anisotropic planes with 180 degrees rotation symmetry which rotate uniformly with displacement perpendicular to the planes so that the pitch is equal to twice the period. The sinusoidal modulation of the structure which possesses double-helix symmetry results in a single band gap for co-handed light with the same sense of circular polarization as the handedness of the helical structure. Orthogonally polarized light is freely transmitted. Within the band gap the wavelength in the medium equals the structure pitch. Double-helix symmetry may also be implemented into a fiber geometry by twisting glass optical fiber with noncircular core cross section as it passes through a miniature oven. In addition to the polarization-selective resonant band observed in planar chiral gratings, we observe two additional modes of optical interaction when the pitch exceeds the wavelength in the fiber. In chiral long period gratings, dips in transmission are observed at wavelengths associated with coupling of the core mode and distinct cladding modes mediated by the chiral grating. In chiral intermediate period gratings, a broad scattering band is observed due to scattering out of the fiber into a continuum of states. Gratings with uniform pitch as well as with a specially designed pitch profile can be utilized to produce a variety of polarization selective devices. In addition to describing optical chiral gratings, we describe studies of microwave planar and fiber gratings, which played a key role in the development of optical fiber chiral gratings.
We have produced chiral fiber Bragg gratings with double-helix symmetry and measured the polarization and wavelength selective transmission properties of these structures. These gratings interact only with circularly polarized light with the same handedness as the grating twist and freely transmit light of the orthogonal polarization. The optical characteristics of chiral fibers are compared to those of planar cholesteric structures. The resonant standing wave at the band edge or at a defect state within the band gap, as well as the evanescent wave within the band gap is comprised of two counterpropagating components of equal amplitude. The electric field vector of such a circularly polarized standing wave does not rotate in time; rather it is linearly polarized in any given plane. The standing wave may be described in terms of the sense of circular polarization of the two counterpropagating components. The wavelength dependence of the angle q between the linearly polarized electromagnetic field and the extraordinary axis, which is constant throughout a long structure, is obtained in a simple calculation. The results are in good agreement with scattering matrix calculations. Resonant chiral gratings are demonstrated for microwave radiation whereas chiral gratings with pitch exceeding the wavelength are demonstrated at optical wavelengths in single-mode glass fibers. The different functionalities of these fibers are discussed.
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