All-Optical/Mechanical Circuit
A significant step in developing an all
optical technology was reported in 1994 by Kuzyk and Welker, who
demonstrated all 5 all-optical device classes in an all-optical
position stabilizer.[1] The position stabilizer, diagramed below, keeps
the position of the hanging mirror fixed, and operates as follows.
Light from a laser is launched into an interferometer
(dashed box). One of the mirrors of the interferometer hangs from a
photomechanical optical fiber, whose length change is proportional to
the light intensity. The output of the interferometer is launched into
the hanging fiber, thus providing optical feedback, as follows.
The intensity of light leaving the interferometer
depends on the length of the fiber. So, if the length of the fiber
changes due to an external agent, so does the intensity leaving the
interferometer. This intensity change causes the length of the fiber to
activity change to oppose the external agent, thus preventing the
hanging mirror from moving.
Welker and Kuzyk found that a 30cm fiber could be
actively kept constant to within about 3 nm, or one part in 108.
The fiber also acted as a positioner - the length of the fiber could be
changed by varying the laser power; and, as a digital positioner - by
applying mechanical shocks to the system, it would jump through several
discrete length states, thus acting mechanically as does a digital
electronic device.
To summarize, in this device (1) information about the
fiber length is transmitted on a beam of light (optical transmission);
(2) the fiber length changes length in response to light (optical
actuation); (3) the light beam senses the position of the mirror
(optical sensor); (4) the interferometer determines the required change
in intensity to resist external stress (optical logic); and (5) the
whole optical circuit is powered by a laser. Thus, this device
demonstrates that an all-optical device technology is possible.
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Photomechanical Optical Devices (PODs)
A breakthrough was made in 1995 when the
all-optical/mechanical device was miniaturized into a sub-milimeter
device[2] whose operation is identical to the all-optical/mechanical
circuit. The device, made from a short segment of a polymer optical
fiber is diagrammed below.
One method for making mirrors are to burn Bragg gratings
onto the two ends of the fiber (shown as stripes) using a high powered
laser. A bragg grating is a series of parallel planes of elevated or
depressed refractive index. Each of these planes reflects a small
portion of the light, but when many such planes are taken together, the
reflectivity can approach 100%.
An important property of a Bragg grating is that it
reflects only the color of light whose wavelength is properly matched
to the period of the grating. The interferometer formed by the two
reflectors controls the light's intensity within. Since the material is
photomechanical, the logic, sensing and actuation is all contained
within this basic element, called a Photomechanical Optical Device
(POD).
A POD has two inputs and two outputs. The light power
that exits a POD depends on the input power, the externally applied
stress, and its mechanical and optical history (two POD's that have
been exposed to different light levels and stresses will behave
differently). Thus, we can think of a POD as a neuron in the brain with
the added features that it acts as a sensor and is an actuator.
[1] D. J. Welker and M. G. Kuzyk, "Photomechanical
Stabilization in a Polymer Fiber-Based All-Optical Circuit," Appl.
Phys. Lett. 64, 809 (1994).
[2] D. J. Welker and M. G. Kuzyk, "Optical and
Mechanical Multistability in a Dye-Doped Polymer Fiber Fabry-Perot
Waveguide," Applied Phys. Lett. 66, 2792 (1995).
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