Self-aligning universal beam coupler
David A. B. Miller
Ginzton Laboratory, Stanford University, 348 Via Pueblo Mall, Stanford CA 94305-4088, USA
Abstract: We propose a device that can take an arbitrary monochromatic input beam and couple it into a
single-mode guide or beam. Simple feedback loops from detectors to modulator elements allow the device
to adapt automatically to any specific input beam form. Potential applications include automatic
compensation for misalignment and defocusing of an input beam, coupling of complex modes or multiple
beams from fibers or free space to single-mode guides, and retaining coupling to a moving source.
Straightforward extensions allow multiple different overlapping orthogonal input beams to be separated
simultaneously to different single-mode guides with no splitting loss in principle. The approach is suitable
for implementation in integrated optics platforms that offer elements such as phase shifters, Mach-Zehnder
interferometers, grating couplers, and integrated monitoring detectors, and the basic approach is applicable
in principle to other types of waves, such as microwaves or acoustics.
There has recently been growing interest in exploiting multiple modes, both in optical fibers  and free-space ,
for expanding communications bandwidth and capabilities. Selecting and coupling to complicated mode forms such
as higher fiber modes  or angular momentum beams  is challenging, however, especially if splitting losses are
to be avoided. Coupling to waveguides generally remains difficult in optics, especially if alignment or precise
focusing cannot be guaranteed. Simultaneous coupling of multiple overlapping input modes without splitting loss
has had few known solutions [3,4]. Here we propose a novel approach to coupling that both allows complicated
modes to be coupled, e.g., to single-mode waveguides, and can accommodate misalignments and movements, all
without moving parts. It can also couple multiple different overlapping modes simultaneously to different output
waveguides, without fundamental splitting loss. The approach can be implemented using standard integrated optical
components, detectors and simple local feedback loops, and is automatic, not requiring advance knowledge of the
beam form to be coupled. The approach could be applied in principle to other waves, such as radio waves,
microwaves or acoustics .
2. Device concept
Fig. 1 shows a conceptual schematic of the approach. For simplicity for the moment, we consider a beam varying
only in the lateral direction. For illustration we divide the arbitrary input beam into 4 pieces, each incident on a
different one of the 4 beam splitter blocks. Each block includes a variable reflector (except number 4, which is 100%
reflecting) and a phase shifter. (The phase shifter PS1 is optional, allowing the overall output phase of the beam to
be controlled.) We presume loss-less devices whose reflectivity and phase shift can be set independently, for
example, by applied voltages for electrooptic or thermal control. For the moment, we neglect diffraction inside the
optics and presume that the phase shifters, reflectors, and detectors operate equally on the whole beam going
through one beamsplitter.
We shine the input beam onto the beamsplitter blocks as shown. Initially, the phase shifter and reflectivity
settings can be arbitrary as long as the reflectivities are non-zero so that we start with non-zero powers on the
detectors. First, we adjust the phase shifter P4 to minimize the power on detector D3. Doing so ensures that the wave
reflected downwards from beamsplitter 3 is in antiphase with any wave transmitted from the top through
beamsplitter 3. Then we adjust the reflectivity R3 to minimize the power in detector D3 again, now completely
cancelling the transmitted and reflected beams coming out of the bottom of beamsplitter 3. (If there are small phase
changes associated with adjusting reflectivity, then we can iterate this process, adjusting the phase shifter again, then
the reflectivity, and so on, to minimize the D3 signal.)
We then repeat this procedure for the next beamsplitter block, adjusting first phase shifter P3 to minimize the
power in detector D2, and then reflectivity R2 to minimize the D2 signal again. We repeat this procedure along the
line of phase shifters, beamsplitters and detectors. Finally, all the power in the incident beam emerges from the
output port on the right. (This approach could also be used to combine multiple beams of unknown relative phases,
as in fiber laser systems , with each beam incident on a separate beamsplitter block.)
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