Delay line interferometer multiplexer

1. A delay line interferometer (DLI) multiplexer (MUX), comprising:

a first stage comprising:

a first DLI including a first left input, a first right input, and a first output and having a free spectral range (FSR) that is about four times a nominal channel spacing; and

a second DLI including a second left input, a second right input, and a second output and having an FSR that is about four times the nominal channel spacing, wherein transmission peaks of the second DLI are shifted relative to transmission peaks of the first DLI by an amount about equal to the nominal channel spacing; and a second stage coupled to the first stage and comprising a third DLI including:

a third left input optically coupled to the first output of the first DLI;

a third right input optically coupled to the second output of the second DLI; and

a third output;

wherein an FSR of the third DLI is about two times the nominal channel spacing.

2. The DLI MUX of claim 1 , wherein each of the first, second and third DLIs comprises:

a 2×2 multi-mode interference (MMI) device including: a respective one of the first, second or third left input, a respective one of the first, second or third right input, a left output, and a right output;

an MMI combiner including: a left input, a right input, and a respective one of the first, second or third output;

a first optical path coupled between the left output of the 2×2 MMI device and the left input of the MMI combiner; and

a second optical path having a different length than the first optical path, wherein the second optical path is coupled between the right output of the 2×2 MMI device and the right input of the MMI combiner.

3. The DLI MUX of claim 1 , wherein each of the first, second and third DLIs is configured to apply:

a left transmission function to a corresponding optical signal received at a respective one of the first, second or third left inputs; and

a right transmission function to a corresponding optical signal received at a respective one of the first, second or third right inputs.

4. The DLI MUX of claim 3 , wherein:

the right transmission function of each of the first and second DLIs is offset from the left transmission function of each of the first and second DLIs by about two times the nominal channel spacing;

the right transmission function of the third DLI is offset from the left transmission function of the third DLI by about the nominal channel spacing.

5. The DLI MUX of claim 4 , wherein:

the first DLI is configured to receive at the first left input a first optical signal having a first wavelength;

the left transmission function of the first DLI has a transmission peak aligned to the first wavelength;

the first DLI is configured to receive at the first right input a second optical signal having a second wavelength;

the right transmission function of the first DLI has a transmission peak aligned to the second wavelength; the second DLI is configured to receive at the second left input a third optical signal having a third wavelength;

the left transmission function of the second DLI has a transmission peak aligned to the third wavelength;

the second DLI is configured to receive at the second right input a fourth optical signal having a fourth wavelength;

the right transmission function of the second DLI has a transmission peak aligned to the fourth wavelength;

the third DLI is configured to receive at the third left input a first multiplexed optical signal including the first and second optical signals;

the left transmission function of the third DLI has one transmission peak aligned to the first wavelength and another transmission peak aligned to the second wavelength;

the third DLI is configured to receive at the third right input a second multiplexed optical signal including the third and fourth optical signals;

the right transmission function of the third DLI has one transmission peak aligned to the third wavelength and another transmission peak aligned to the fourth wavelength; and

the third DLI is configured to output at the third output a third multiplexed signal including the first, second, third, and fourth optical signals.

6. The DLI MUX of claim 1 , wherein the DLI MUX is configured to receive four optical signals having wavelengths spaced at about the nominal channel spacing at, respectively, the first left input, the first right input, the second left input and the second right input and to output a single multiplexed optical signal including the four optical signals at the third output.

7. The DLI MUX of claim 1 , further comprising:

a third stage coupled to the first stage and comprising fourth, fifth, sixth and seventh DLIs;

wherein:

each of the fourth, fifth, sixth and seventh DLI includes a left input, a right input, and an output;

an output of the fourth DLI is coupled to the first left input of the first DLI;

an output of the fifth DLI is coupled to the first right input of the first DLI;

an output of the sixth DLI is coupled to the second left input of the second DLI;

an output of the seventh DLI is coupled to the second right input of the second DLI; and

an FSR of each of the fourth, fifth, sixth and seventh DLI is about eight times the nominal channel spacing.

8. The DLI MUX of claim 7 , wherein the DLI MUX is configured to receive eight optical signals having wavelengths spaced at about the nominal channel spacing at, respectively, the left and right inputs of the fourth, fifth, sixth and seventh DLIs and to output a single multiplexed optical signal including the eight optical signals at the third output of the third DLI.

9. A delay line interferometer (DLI) multiplexer (MUX), comprising:

a first stage comprising:

a first DLI configured to receive first and second optical signals having respective first and second wavelengths and further configured to output a first multiplexed signal including the first and second optical signals; and

a second DLI configured to receive third and fourth optical signals having respective third and fourth wavelengths and further configured to output a second multiplexed signal including the third and fourth optical signals;

wherein a free spectral range (FSR) of each of the first and second DLIs is about four times a nominal channel spacing of the first, second, third and fourth optical signals and wherein transmission peaks of the second DLI are shifted relative to transmission peaks of the first DLI by an amount about equal to the nominal channel spacing; and

a second stage coupled to the first stage and comprising a third DLI configured to receive the first and second multiplexed signals and further configured to output a third multiplexed signal including the first, second, third, and fourth optical signals.

10. The DLI MUX of claim 9 , wherein the first DLI, second DLI and third DLI are monolithically integrated on a substrate having a footprint of less than 300 micrometers by 100 micrometers.

11. The DLI MUX of claim 9 , wherein:

the first DLI comprises:

a first 2×2 multi-mode interference (MMI) device including a first left input, a first right input, a left output and a right output;

a first MMI combiner including a left input, a right input, and a first output;

a first optical path coupled between the left output of the first 2×2 MMI device and the left input of the first MMI combiner; and

a second optical path having a different length than the first optical path, wherein the second optical path is coupled between the right output of the first 2×2 MMI device and the right input of the first MMI combiner;

the second DLI comprises:

a second 2×2 MMI device including a second left input, a second right input, a left output and a right output;

a second MMI combiner including a left input, a right input, and a second output;

a third optical path coupled between the left output of the second 2×2 MMI device and the left input of the second MMI combiner; and

a fourth optical path having a different length than the third optical path, wherein the fourth optical path is coupled between the right output of the second 2×2 MMI device and the right input of the second MMI combiner; and

the third DLI comprises:

a third 2×2 MMI device including a third left input, a third right input, a left output and a right output;

a third MMI combiner including a left input, a right input, and a third output;

a fifth optical path coupled between the left output of the third 2×2 MMI device and the left input of the third MMI combiner; and

a sixth optical path having a different length than the fifth optical path, wherein the fifth optical path is coupled between the right output of the third 2×2 MMI device and the right input of the third MMI combiner.

12. The DLI MUX of claim 11 , wherein:

the first 2×2 MMI device is configured to apply an unequal splitting ratio to each of the first and second optical signals; and

the second 2×2 MMI device is configured to apply an unequal splitting ratio to each of the third and fourth optical signals.

13. The DLI MUX of claim 12 , wherein each of the first and second 2×2 MMI devices comprises a butterfly MMI device.

14. A monolithic chip, comprising:

a delay line interferometer (DLI) multiplexer (MUX), comprising:

a first stage comprising:

a first DLI including a first left input, a first right input, and a first output and having a free spectral range (FSR) that is about four times a nominal channel spacing; and

a second DLI including a second left input, a second right input, and a second output and having an FSR that is about four times the nominal channel spacing, wherein transmission peaks of the second DLI are shifted relative to transmission peaks of the first DLI by an amount about equal to the nominal channel spacing; and

a second stage comprising a third DLI including:

a third left input optically coupled to the first output of the first DLI;

a third right input optically coupled to the second output of the second DLI; and

a third output;

wherein an FSR of the third DLI is about two times the nominal channel spacing; and

a plurality of optical signal sources including at least first, second, third and fourth optical signal sources each configured to emit an optical signal having a respective first, second, third or fourth wavelength into a respective one of the first left input, first right input, second left input or second right input.

15. The monolithic chip of claim 14 , further comprising a substrate having substantially rectangular shape, wherein each of the plurality of optical signal sources is located near a respective corner of the substrate and wherein a footprint of the substrate is about 800 micrometers by about 800 micrometers.

16. The monolithic chip of claim 14 , wherein:

the first DLI includes a first 2×2 multi-mode interference (MMI) device, a first MMI combiner, and a long optical path and a short optical path coupled between the first 2×2 MMI device and the first MMI combiner;

the second DLI includes a second 2×2 MMI device, a second MMI combiner, and a long optical path and a short optical path coupled between the second 2×2 MMI device and the second MMI combiner; and

the long optical path of each of the first and second DLI is about 2.5 millimeters longer than the short optical path of the respective first or second DLI.

17. The monolithic chip of claim 16 , wherein each of the first and second optical signal sources is formed in the monolithic chip within a loop formed by the long path of the first DLI and each of the third and fourth optical signal sources is formed in the monolithic chip within a loop formed by the long path of the second DLI.

18. The monolithic chip of claim 14 , wherein transmission functions of the first, second and third DLI are aligned with respect to the first, second, third and/or fourth wavelengths so as to enhance an extinction ratio of each of the optical signals emitted by the first, second, third and fourth optical signal sources, each of the optical signals being a frequency modulated optical signal.

19. The monolithic chip of claim 18 , wherein for each of the optical signals, 1 bits have a first frequency and 0 bits have a second frequency and at least one of the transmission functions is aligned to attenuate the second frequency more than the first frequency.

20. The monolithic chip of claim 14 , wherein transmission functions of the first, second and third DLI are aligned with respect to the first, second, third and/or fourth wavelengths so as to reduce an extinction ratio of each of the optical signals emitted by the first, second, third and fourth optical signal sources, each of the optical signals being a frequency modulated optical signal.

21. The monolithic chip of claim 20 , wherein for each of the optical signals, 1 bits have a first frequency and 0 bits have a second frequency and at least one of the transmission functions is aligned to attenuate the first frequency more than the second frequency.