TECHNICAL FIELD

This disclosure relates generally to fiber optic communication. More particularly, the disclosure relates to a wavelength selective fiber optic switch.

BACKGROUND

This section provides background information related to the present disclosure which is not necessarily prior art.

Modern fiber optic communication systems send data over many different channels corresponding to different optical wavelengths. To control individual channels, such as to switch a channel on and off, high-fidelity wavelength-selective switches are needed in order to control the desired channel without affecting the channels at other wavelengths.

To achieve this, conventional wavelength-selective optical switches typically comprise separate physical devices that are inserted into the optical path. The conventional approaches can be convoluted, expensive, physically large, challenging to integrate and come with various performance tradeoffs (insertion loss, tunability challenges and the like).

SUMMARY

The disclosed technology takes a different approach to providing a wavelength-selective optical switch. The optical switch is small in size and offers low insertion loss. The optical fiber is fashioned by removing a portion of the cladding, as by polishing, abrading or other removal processes, to define an access window through which the air glass interface is brought nearer the core. Within the window region, the evanescent field is accessible.

A grating, sized to moveably fit into the access window, is provided with a predetermined grating pitch corresponding to the wavelength desired to be switched. The grating and optical fiber are physically joined by a fixture that supports relative movement between fiber and grating between a first position in which the grating contacts the core, or is otherwise positioned proximate to the core, such that the grating electromagnetically interacts with the evanescent field of the core and a second position in which the grating is spaced apart from the core.

While a range of mechanical movement solutions are possible, the fixture may be provided with an electrically actuated piezoelectric piston to move between the first and second positions.

When switched to the first position, the grating substantially interacts with the evanescent field. As a result of this interaction, the optical power at the design wavelength is prevented from propagating further in the forward direction—effectively switching OFF the optical signal at the design wavelength.

When switched to the second position, the grating is substantially non interacting with the evanescent field within the window. In this state, the optical power, at all wavelengths present, propagates through the optical fiber with near zero insertion loss. The energy in the evanescent field, though outside of the fiber core, remains bound to the fiber mode and does not result in any power attenuation unless perturbed by an externally introduced object.

Therefore, in one respect the wavelength selective fiber optic switch employs an optical fiber having a fiber core and a cladding. A portion of the cladding is removed to define a window that supports access to the evanescent field that is present when optical power is propagating through the optical fiber.

The switch also employs a grating configured to define a grating wavelength (A), and an adjustable positioning fixture configured to hold the grating proximate to the window and operable to change the relative spacing of the fiber core and grating, between the first and second position. In the first position the grating is held proximate to the fiber core and substantially interacts with the evanescent field. In the second position the grating is held apart from the fiber core and substantially eliminates interaction with the evanescent field.

The cladding-removed portion of the fiber core and the grating cooperatively define a switch that operates in the first position, to substantially impede propagating optical power in the optical fiber at the grating wavelength; and in the second position, to permit propagating optical power in the optical fiber at the grating wavelength.

In another respect, a method is disclosed for switching an optical signal carried by optical energy propagating through an optical fiber having a core and a cladding.

Propagating optical energy is introduced into an optical fiber that has a portion of the cladding removed to define a window for accessing the evanescent field when the optical energy is present in the optical fiber. The optical energy is modulated at a predefined wavelength to carry the optical signal. The relationship between optical wavelength and grating periodicity is expressed in the Bragg equation:
λB=2Λneff

Here λBis the Bragg wavelength (our selected wavelength), Λ is the period or pitch of the physical grating, and neffis the effective index of refraction. Note that the above equation is for the first order grating response, higher orders exist and could also be used in this application. A grating having a defined period is (a) selectively introduced into the evanescent field, such that interaction between the grating and the evanescent field results in suppression of power at the Bragg wavelength from the transmitted spectra. The grating is (b) selectively moved substantially out of the evanescent field such that substantial interaction between the grating and the evanescent field does not occur at the Bragg, or other wavelengths. Through such selective movement of the grating into and out of proximity with the evanescent field the optical signal is switched OFF when the grating is interacting and switched ON when the grating is not interacting.

