Free space variable optical attenuator with integrated input/output power monitors
A free space variable optical attenuator (VOA) utilizes a beamsplitter to create tap beams (of both the input signal and the beam-steered output signal) that are directed into monitoring photodiodes. The beamsplitter is configured to exhibit a non-equal splitting ratio such that the tap beams are only a relatively small portion of the input/output beams. The free space configuration eliminates the need for fiber-based couplers, splices and connections to external monitors, as required in prior art VOA monitoring systems. The VOA utilizes a voltage-controlled, MEMS-based tilt mirror to provide beam steering of the propagating, free space beam in a known manner to introduce attenuation (power reduction) in the output signal.
1. A free space variable optical attenuator with integrated power monitoring comprising
a MEMS-based adjustable position tilt mirror for providing beam steering of an input free space beam to create a beam-steered output free space beam, the degree of steering defined by the adjustable position of the tilt mirror, the input free space beam arriving along an input signal path and the beam-steered output free space beam exiting along an output signal path, the input and output, signal paths terminating along a common plane P of the free space variable optical attenuator;
a single beamsplitter in which both of the input and the beam-steered output free space beams pass therethrough, the single beamsplitter for dividing each of the input and the beam-steered output free space beams into a major portion passing through the single beamsplitter and a minor portion re-directed along an orthogonal monitoring path, the single beamsplitter disposed in optical alignment with the MEMS-based tilt mirror so as to direct the major portion of the beam-steered output free space beam into the output signal path at the common plane P; and
a monitoring arrangement responsive to the minor portions of the input free space beam and the beam-steered output free space beam for measuring input and output power levels to determine the amount of attenuation created by the position of the tilt mirror.
2. The free space variable optical attenuator as defined in claim 1 wherein the monitoring arrangement comprises
an input monitoring arrangement disposed to intercept the re-directed, minor portion of the input free space beam, the input monitoring arrangement including an input monitoring photodiode, where a measurement of optical power received by the input monitoring photodiode is indicative of the input power received by the variable optical attenuator; and
an output monitoring arrangement disposed to intercept the re-directed, minor portion of the beam-steered output free space beam, the output monitoring arrangement including an aperture selected to exhibit a known, constant relationship to an output path from the variable optical attenuator and an output monitoring photodiode disposed in alignment with the aperture such that a measurement of optical power of the beam-steered output free space beam passing through the aperture and received by the monitoring output photodiode, is indicative of the output power exiting the variable optical attenuator.
3. The free space variable optical attenuator as defined in claim 2 wherein the output monitoring arrangement comprises
a pin hole element disposed between the single beamsplitter and the output monitoring photodiode to intercept the minor portion of the beam-steered output free space beam re-directed by the single beamsplitter, the pin hole element formed to include the aperture through which the minor portion of the beam-steered output free space beam passes prior to reaching the output monitoring photodiode.
4. The free space variable optical attenuator as defined in claim 2 wherein the output monitoring arrangement comprises
an absorbent coating material disposed on an outer perimeter of an active region of the output monitoring photodiode, the boundaries of the absorbent coating material defining the aperture of the output monitoring arrangement.
5. The free space variable optical attenuator as defined in claim wherein the monitoring arrangement comprises a split detector configuration disposed to receive the minor portion of the beam-steered output free space beam re-directed by the beam splitter, the split detector configuration comprising
a pair of monitoring photodiodes disposed in a side-by-side relationship, a first monitoring photodiode of the pair of photodiodes measuring a power level associated with the amount of beam-steered output power steered away from a defined aperture, and a second monitoring photodiode of the pair of photodiodes measuring a power level associated with the amount of beam-steered output power passing through a defined aperture; and
a combiner coupled to outputs from the first and second monitoring photodiodes, wherein a combination of measured powers is indicative of the input power applied to the variable optical attenuator.
6. The free space variable optical attenuator as defined in claim 2 , wherein the input monitoring arrangement further comprises
a focusing lens disposed between the single beamsplitter and the input monitoring photodiode.
7. The free space variable optical attenuator as defined in claim 6 , wherein the input monitoring arrangement further comprises
an optical isolator disposed in the free space beam path between the single beamsplitter and the focusing lensing, the optical isolator for preventing reflected free space beams from re-entering the single beamsplitter.
8. The free space variable optical attenuator as defined in claim 2 , wherein the output monitoring arrangement further comprises a focusing lens disposed between the single beamsplitter and the aperture.
9. The free space variable optical attenuator as defined in claim 2 , wherein the attenuator further comprises
a cylindrical lens element disposed in alignment with the common plane P for receiving an input optical signal from the input signal path and collimating the input optical signal, to create the free space input beam applied as an input to the single beamsplitter, the cylindrical lens element also for receiving the beam-steered output free space beam passing through the single beamsplitter and focusing the beam-steered output free space beam to form a focused output free space beam thereafter coupled into the output signal path.
10. The free space variable optical attenuator as defined in claim 9 wherein the attenuator further comprises
a dual-core optical component disposed at an endface of the cylindrical lens element, a first core of the dual-core optical component for propagating the received input optical signal and coupling the propagating input optical signal into the cylindrical lens, and a second core of the dual-core optical component for receiving the focused output free space beam, wherein the diameter of the second core is related to, the diameter of the output monitoring arrangement aperture so that a selected amount of the power of the focused output free space beam is coupled into the second core to create the attenuated optical signal.