Castor Optics’ Photonic lanterns are fiber optic devices that efficiently convert and couple different spatial modes of light between a multimode and single-mode fibers. Mode selectivity in photonic lanterns ensures that specific modes are transmitted to corresponding single mode fibers.
Applications
Single mode Performance
The Photonic Lantern was originally developed for astrophysics to balance the efficiency of multimode collection optics for starlight with the high performance of single-mode components, such as fiber Bragg gratings. This concept remains highly relevant today, particularly in quantum optics. For instance, superconducting nanowire single-photon detectors operate efficiently in the single-mode regime, even though most collected signals are few-mode or multimode. The photonic lantern effectively bridges this gap, enabling single-mode performance with multimode signals by converting higher-order modes to the fundamental mode, allowing them to be projected on individual detectors.
Coherent Few Mode Detection
A limiting factor in many coherent detection systems is the requirement to operate in the single-mode regime. This is particularly true for technologies like optical coherence tomography and LiDAR. However, light scattered from a target is not always returned in the fundamental mode. For scatterers that return at wider angles, the collected light is often better coupled into higher-order modes. The photonic lantern is ideally suited to excite, collect, and coherently detect these signals, as it operates in a few-mode regime only at the distal end of the probe, increasing the signal-tonoise ratio of the system. Additionally, the photonic lantern can illuminate targets in higher-order modes, enabling novel sensing applications.
Spatial Mode Multiplexing
With photonic lanterns supporting an ever-higher number of modes, a promising application is few-mode communication, increasing information throughput in optical fibers by encoding information in different spatial modes. These modes can propagate simultaneously with minimal mixing if an appropriate fiber is chosen. Similarly, quantum states of light can be encoded in distinct spatial modes, or different modes can be used to generate novel quantum states. In fact, the latter approach has been proposed to generate non-Gaussian single-photon triplets. Finally, photonic lanterns can superpose spatial modes to create various excitation patterns without using cumbersome spatial light modulators for applications in microscopy and endoscopy. Reciprocally, the photonic lantern can be used as a spatial mode demultiplexer, allowing low-loss characterization of in-cavity spatial modes of fiber lasers.
General Specifications
- Wavelength Range: 1250-1600 nm
- Insertion Loss: ≤0.2 dB (Typical)
- Optical Return Loss: ≥20 dB
- Isolation: ≥28 dB
- Single-mode fiber type: SMF-28
Working Principles
Photonic lanterns are a family of waveguide devices that facilitate bidirectional light transmission between a multimode waveguide and a set of waveguides supporting fewer modes. Photonic lanterns from Castor Optics use step-index optical fibers as waveguides. Due to the cylindrical symmetry of optical fibers, the spatial propagation modes of light are described by scalar LP modes, solutions to the Bessel function. The fundamental Gaussian mode is LP01, and the first-order modes are the twice-degenerate LP11 modes, and so on. As shown in Fig. 1, input single-mode fibers exhibits a fundamental LP01 Gaussian mode whereas the few-mode structure (here on the right side) can support a combination of LP01 and LP11 modes. Each of these modes has a one-to-one correspondence with a specific single-mode fiber.
In practical terms, the lantern single-mode fibers are simply SMF-28 structures that can be connectorized or spliced for low-loss transmission. The few-mode throughput can be adapted to suit the desired use case. For applications where the modes propagate in free space, the few-mode throughput acts as the distal end of a probe. For example, given the ultra-short nature of the device, the lantern can be embedded in a standard 12.7mm ferrule. This ferrule can be integrated into the collection optics of medical probes or LiDAR systems. For enhanced mechanical integrity, the lantern can also be placed inside an FC connector recessed into a metal tube, as shown in Figure 1. Finally, if the modes are to be fiber-propagated, the photonic lantern can be optimized to couple into a customer-chosen few-mode fiber.
Figure 1: Schematic of a packaged two-mode selective photonic lantern. The left side shows the input single-mode fibers with LP01 modes. The right side depicts the output with combined LP01 and LP11 modes. The optical structure sits in the metal housing on the right side.
Écrit par Joseph Lamarre, Scientifique d'application et
Kathy Beaudette, Directrice du développement des affaires
