We report the design and fabrication of three-dimensional integrated mode couplers operating in the C-band. These mode-selective couplers were inscribed into a boro-aluminosilicate photonic chip ...using the femtosecond laser direct-write technique. Horizontally and vertically written two-core couplers are shown to allow for the multiplexing of the LP11a and LP11b spatial modes of an optical fiber, respectively, with excellent mode extinction ratios (25-37 + dB) and low loss (~1 dB) between 1500 and 1580 nm. Furthermore, optimized fabrication parameters enable coupling ratios close to 100%. When written in sequence, the couplers allow for the multiplexing of all LP01, LP11a and LP11b modes. This is also shown to be possible using a single 3-dimensional three-core coupler. These integrated mode couplers have considerable potential to be used in mode-division multiplexing for increasing optical fiber capacity. The three-dimensional capability of the femtosecond direct-write technique provides the versatility to write linear cascades of such two- and three-core couplers into a single compact glass chip, with arbitrary routing of waveguides to ensure a small footprint. This technology could be used for high-performance, compact and cost-effective multiplexing of large numbers of modes of an optical fiber.
The use of the Type I and Type II scheme, first introduced and used by fiber Bragg grating researchers, has recently been adopted by the ultrafast laser direct-write photonics community to classify ...the physical geometry of waveguides written into glasses and crystals. This has created confusion between the fiber Bragg grating and direct-write photonics community. Here we propose a return to the original basis of the classification based on the characteristics of the material modification rather than the physical geometry of the waveguide.
Integrated photonics promises solutions to questions of stability, complexity, and size in quantum optics. Advances in tunable and non-planar integrated platforms, such as laser-inscribed photonics, ...continue to bring the realisation of quantum advantages in computation and metrology ever closer, perhaps most easily seen in multi-path interferometry. Here we demonstrate control of two-photon interference in a chip-scale 3D multi-path interferometer, showing a reduced periodicity and enhanced visibility compared to single photon measurements. Observed non-classical visibilities are widely tunable, and explained well by theoretical predictions based on classical measurements. With these predictions we extract Fisher information approaching a theoretical maximum. Our results open a path to quantum enhanced phase measurements.
ABSTRACT
The characterization of exoplanets is critical to understanding planet diversity and formation, their atmospheric composition, and the potential for life. This endeavour is greatly enhanced ...when light from the planet can be spatially separated from that of the host star. One potential method is nulling interferometry, where the contaminating starlight is removed via destructive interference. The GLINT instrument is a photonic nulling interferometer with novel capabilities that has now been demonstrated in on-sky testing. The instrument fragments the telescope pupil into sub-apertures that are injected into waveguides within a single-mode photonic chip. Here, all requisite beam splitting, routing, and recombination are performed using integrated photonic components. We describe the design, construction, and laboratory testing of our GLINT pathfinder instrument. We then demonstrate the efficacy of this method on sky at the Subaru Telescope, achieving a null-depth precision on sky of ∼10−4 and successfully determining the angular diameter of stars (via their null-depth measurements) to milliarcsecond accuracy. A statistical method for analysing such data is described, along with an outline of the next steps required to deploy this technique for cutting-edge science.
Alkali-free borosilicate glasses are one of the most used dielectric platforms for ultrafast laser inscribed integrated photonics. Femtosecond laser written waveguides in commercial Corning Eagle ...2000, Corning Eagle XG and Schott AF32 glasses were analyzed. They were studied in depth to disclose the dynamics of waveguide formation. We believe that the findings presented in this paper will help bridge one of the major and important gaps in understanding the ultrafast light-matter interaction with alkali-free boroaluminosilicate glass. It was found that the waveguides are formed mainly due to structural and elemental reorganization upon laser inscription. Aluminum along with alkaline earth metals were found to be responsible for the densification and silicon being the exchanging element to form a rarefied zone. Strong affinity towards alkaline earth elements to form the densified zone for waveguides written with high feed rate (>200 mm/min) were identified and explained. Finally we propose a plausible solution to form positive refractive index change waveguides in different glasses based on current and previous reports.
Nulling interferometry enables astronomers to advance beyond the resolving power of ground-based telescopes with the goal of directly detecting exo-planets. By diminishing the overwhelming emission ...of the host star through destructive interference, radiation from young companions can be observed. The atmospheric transmission window centered around 4 μm wavelength is of particular interest because it has a favorable contrast between star and planet as well as a reduced atmospheric disturbance. For robustness and high stability, it is desirable to employ integrated devices based on optical waveguide technology. Their development is hindered at this wavelength range due to the lack of suitable host materials and compatible fabrication techniques to create low-loss photonic devices. This paper details our work on femtosecond laser direct-written optical waveguides and key components for an on-chip nulling interferometer inside gallium lanthanum sulphur glass. By combining cumulative heating fabrication with the multiscan technique, single-mode optical waveguides with propagation losses as low as 0.22 ± 0.02 dB/cm at 4 μm and polarization-dependent losses of < 0.1 dB/cm were realized. Furthermore, S-bends with negligible bending loss and broadband Y-splitters with 50/50 power division across a 600 nm wavelength window (3.6 - 4.2 μm) and low losses of < 0.5 dB are demonstrated. Directional couplers with an equal splitting ratio complement these main building blocks to create a future compact nulling interferometer with a total projected intrinsic loss of < 1 dB, a value that is sufficient to perform future on-sky experiments in relatively short observation runs on ground-based telescopes.
There are numerous advantages to exploiting diffraction-limited instrumentation at astronomical observatories, which include smaller footprints, less mechanical and thermal instabilities and high ...levels of performance. To realize such instrumentation it is imperative to convert the atmospheric seeing-limited signal that is captured by the telescope into a diffraction-limited signal. This process can be achieved photonically by using a mode reformatting device known as a photonic lantern that performs a multimode to single-mode transition. With the aim of developing an optimized integrated photonic lantern, we undertook a systematic parameter scan of devices fabricated by the femtosecond laser direct-write technique. The devices were designed for operation around 1.55 μm. The devices showed (coupling and transition) losses of less than 5% for F/# ≥ 12 injection and the total device throughput (including substrate absorption) as high as 75-80%. Such devices show great promise for future use in astronomy.
We compare and contrast novel techniques for the fabrication of chirped broadband fiber Bragg gratings by ultrafast laser inscription. These methods enable the inscription of gratings with flexible ...period profiles and thus tailored reflection and dispersion characteristics in non-photosensitive optical fibers. Up to 19.5 cm long chirped gratings with a spectral bandwidth of up to 30 nm were fabricated and the grating dispersion was characterized. A maximum group delay of almost 2 ns was obtained for linearly chirped gratings with either normal or anomalous group velocity dispersion, demonstrating the potential for using these gratings for dispersion compensation. Coupling to cladding modes was reduced by careful design of the inscribed modification features.
Photonic technologies will be at the heart of future terrestrial planet hunting interferometers. In particular the mid-infrared spectral region between 3.5 - 4.2 μm is the ideal window for hunting ...for young extra-solar planets, since the planet is still hot from its formation and thus offers a favorable contrast with respect to the parent star compared to other spectral regions. This paper demonstrates two basic photonic building blocks of such an instrument, namely single-mode waveguides with propagation losses as low as 0.29±0.03 dB/cm at a wavelength of 4 μm as well as directional couplers with a constant splitting ratio across a broad wavelength band of 500 nm. The devices are based on depressed cladding waveguides inscribed into ZBLAN glass using the femtosecond laser direct-write technique. This demonstration is the first stepping stone towards the realization of a high transmission mid-infrared nulling interferometer.
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