Compact solid-state variable optical delay line with a large continuous tuning range
In accordance with the invention, a variable delay line with continuous tuning comprises an optical path providing at least one continuous delay element coupled to a series of switchable binary fixed delays. The series of switchable delays can be configured to select a delay among a plurality of incremental delays, and the continuous delay can set a continuous delay in a range encompassing the delay increment of the binary series. In a preferred embodiment, the variable delay element comprises a tunable all pass filter (APF) with delay tunable from essentially 0 to a time T and the switchable binary fixed delays form a series (0,T), (0, 2T), (0, 4T), . . . , [0, (2n+1)], where n is an integer ≧0. The switches are preferably Mach-Zehnder switches. In an alternative embodiment, the continuous delay element comprises a suitable pair of APF arms. The path and all components can be fabricated as an integrated planar waveguide (solid state) device.
1. A variable optical delay line with a large continuous tuning range comprising:
an discretely variable optical delay line for receiving an optical signal, the discretely variable optical delay line comprising a plurality of switchable binary paths configured to provide signal delay paths selected from a sequence of discretely differing delays; and
serially optically coupled with the discretely variable delay line, a continuously variable optical delay line for receiving the optical signal to provide a continuously variable delay from a range of delays substantially encompassing a delay increment in the discretely variable delay line.
2. The variable optical delay line of claim 1 wherein the discretely variable optical delay line is configured to provide a sequence of delays that differ by a delay increment T and the continuously variable optical delay line provides a delay in the continuous range (0,T).
3. The variable optical delay line of claim 1 wherein the discretely variable optical delay line comprises a plurality of switchable binary fixed delays in which one delay is substantially 0 and the other delay is substantially (2n+1) T where T is the delay increment and n is an integer ≧0.
4. The variable optical delay line of claim 3 wherein the discretely variable optical delay line comprises a switchable binary fixed delay of (0,T) serially connected to a switchable binary delay of (0, 2T), where T is the delay element.
5. The variable optical delay line of claim 1 wherein the continuously variable optical delay line comprises a switchable binary delay in which one delay is substantially zero and the other delay is a continuously variable delay in the range (0,T) where T is the delay increment.
6. The variable optical delay line of claim 1 wherein the continuously variable optical delay line comprises a switchable binary delay line in which one delay is a continuously variable delay in the range (0,T), and the other delay is a continuously variable delay in the range (T, 2T), where T is the delay increment.
7. The variable optical delay line of claim 1 wherein the continuously variable optical delay line comprises an all pass filter.