Abstract.
Fiber optic sensors are currently used to measure temperature, pressure,
strain, power, chemical concentrations and more [1]. Evanescent fiber
optic sensing is the most popular. The evanescent tails of guided modes
interact with the surrounding medium. Information about chemicals or
perturbations there are obtained by measuring the change in mode power,
polarization or delay. Key benefits of fiber optic sensors include its
compact size, durability in extreme environments, low power requirements,
and low cost.
Currently, fiber optic sensors do not have control over specific modes,
only large groups [2]. For instance, it is desirable to launch significant
power into higher order modes to increase the sensitivity of the instrument.
But, only one-dimensional knowledge is possible with such limited schemes.
Each spatial mode has a different fraction of its power traveling outside
the fiber core. The penetration depth of each mode is different, and
therefore provides two-dimensional accuracy in measurement. By comparing
the power loss of several modes, radial information about concentration
variations from the core can be calculated.
The goal of this project is to use a novel multiplexing technique to
gain exact control over every spatial mode in optical fibers. Mode-division-multiplexing
(MDM) uses the spatial modes present in optical fiber as an orthogonal
basis. The spatial profiles of multiple modes are stored in a volume
hologram. Individual modes are launched and detected with angle-multiplexed
holograms. Therefore, accurate information of mode attenuatiosn due
to the surrounding medium is known. In addition to sensing applications,
addressing the spatial modes of a multimode fiber (MMF) increases the
bandwidth of an optical communication system [3]. Multiple modes in
the transmission channel provide extra degrees of freedom, and hence
greater capacity [4]. Presently, fiber optic communication systems do
not use the spatial modes to carry information. Modal dispersion decreases
the useable bandwidth of MMF links that do not address the multimode
nature of the channel [5]. This project will also implement the MDM
scheme to increase the bandwidth, and therefore, the speed in MMF communication
systems.
Project Summary. The number of spatial modes that can be supported
by a multimode fiber (MMF) with a core of diameter d is 
where 
is the wavelength
of light, n1 is the index of the core and n2 is
the index of the cladding. Figure 1 is a plot of the number of spatial
modes available at 1=800 nm.
Figure
1.
Number of spatial modes in MMF at 800nm for different core diameters.
For
example, for d=50 um, n1=1.46, and n2=1.45, the number of spatial
modes that can be supported is 1,121. Therefore a multimode fiber represents
a compact and simple medium for transmitting large number of spatial
modes over long distances.
Femtosecond holograms are used to first characterize the spatial mode
structure of a MMF. Multiple holograms will be recorded in a crystal;
each hologram corresponds to a different spatial mode differentiated
from others by the reference beam angle. Next, a single spatial mode
of the fiber will be excited using the phase conjugate of the reference
beam [4]. Coupling between spatial modes will be studied theoretically
and experimentally. A high bandwidth link addresses multiple spatial
modes in parallel. If needed, a coding scheme will be developed to counteract
any mode coupling that may occur. Finally, we envision a 20-channel
MDM system with a total channel capacity of 100Gb/s. While the development
of the communication system is ongoing, we will use the MDM system to
measure concentrations of fluorescently tagged molecules surrounding
a stripped fiber. At first, two modes will be used. One mode will have
a small amount of power outside the core, the other will have significant
power there. The power launched into each mode will be recorded, and
compared to the ouput power of each mode. The percentage of power of
each mode present in the cladding is known. The power lost by each mode
indicates the concentration of molecules present in the path of the
evanescent portion of the mode. In this way, we will measure depth concentration
information and compare to other fiber optic sensors. Mode coupling
is potentially a large problem for such a sensor.
A schematic diagram of the holographic mode coupling method we propose
is shown in Figure 2. A short laser pulse launches multiple spatial
modes in a MMF. The modes are separated from one another due to mode
dispersion after a sufficient propagation distance inside the fiber.
The light emerging from the fiber is imaged onto a second identical
MMF and the system is aligned to maximize coupling efficiency. A holographic
recording material is inserted between the lens and the second fiber.
A hologram of the desired spatial mode is recorded by selecting the
delay of the reference pulse (from the same pulsed laser source) so
that the mode propagating through the fiber and the reference pulse
arrive at the same time on the recording material [5].
