TPF-I SWG Report - Exoplanet Exploration Program - NASA

C HAPTER 4
combiner, two beams are extracted, so a total of four beams are sensed at the array. For the second crosscombiner
a second fringe tracker would be needed unless a laser metrology could be used to transfer
measured phase to the second nuller.
4.6.2 Multi-Axial Beam Combiners
In contrast to classical beamsplitter-based beam-combiner designs, it is also possible to combine optical
beams using single-mode fibers. Technology at long wavelengths has not advanced to the point where
each of the various beams to be combined can be injected into separate fibers and then combined using
cross-couplers. However, another approach is feasible. Separate beams can be directly combined into a
single fiber simply by using a common focusing optic, as in Figure 4-14. Each of the individual beams
must couple to the same spatial mode in the single-mode fiber. Thus, if the beams (in the two beam case)
arrive with a relative phase shift of π radians, the two beams entering the fiber mode will cancel each
other. Note that a large number of beams can be combined in one step, as a number of beams around the
periphery of a common focusing optic can all be simultaneously focused onto the common fiber tip
(Figure 4-15; Karlsson et al. 2004; Wallner et al. 2004). This method of “fiber nulling” has now been
verified in the optical regime, where deep narrowband nulls close to one part in a million have already
been obtained (Haguenauer and Serabyn 2006) , and in the near-infrared, where broad-band nulls of a few
10 -4 have been obtained across the H band (Figure 4-16; Mennesson et al. 2006). However, while simple
and easy to use, this nulling approach also has disadvantages, including lower efficiencies in the case of a
small number of beams, and the lack of a complementary, or “bright” output. Thus, more experience with
this type of combiner is required to fully understand the trade-offs relative to classical beamcombiners.
Focal plane
intensity
Single-mode
waveguide
Fiber fundamental mode
Single sub-pupil intensity
Interference intensity
beam 1
Focusing optics
beam 2
Combined field amplitude
Transversal cuts in focal plane
Figure 4-14. Principle of single-mode fiber beam combination. The two beams are combined by the
same focusing element onto the core of a single-mode fiber.
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