RELATED APPLICATIONS

This is the first patent application related to this matter.

FIELD

The present disclosure is related to methods and devices for optical signal processing, and in particular to methods and devices for variable-bandwidth frequency-based filtering of optical signals.

BACKGROUND

Optical communication encodes data in modulated light waveforms in an optical transmission medium, such as a fiber optic cable. In some communication schemes, the optical transmission medium may carry multiple data channels, each data channel occupying a portion of electromagnetic frequency spectrum of a defined bandwidth. As the spectrum used in optical communication networks gets more and more crowded, efficient utilization of optical frequency bandwidth becomes critical for high-speed, high-volume optical networks. One way to increase channel efficiency is by allowing data transmission bandwidth to be variable based on demand. One of the key components of a bandwidth-tunable optical network (also called an elastic optical network) is the variable bandwidth (VB) filter, whose transmission bandwidth can be dynamically varied on demand to allocate variable amounts of frequency bandwidth to a data channel. Some VB filters may also allow the wavelength of the channel (e.g., the center frequency of the channel) to be varied. However, existing VB filter approaches used for silicon photonics technology have limitations that require trade-offs among factors such as control complexity, cost, reconfigurability, and size. Thus, there exists a need for a simple, low-cost, highly configurable variable-bandwidth filter with a small footprint that also enable wavelength or frequency tuning.

SUMMARY

In various embodiments described herein, methods and devices are disclosed that provide a variable-bandwidth optical filter with frequency tuning. A universal variable bandwidth (VB) optical filter architecture is disclosed, based on microring resonators that can vary both operation wavelength and bandwidth with no extra complexity relative to conventional wavelength tunable filters. In some embodiments, the filter architecture described herein may provide a universal filter design for any arbitrary shape of filter response, such as second-order, fourth-order, sixth-order, and so on. The filter characteristics (e.g., insertion loss, in-band ripple, and out-of-band rejection level) may be maintained over the bandwidth tuning range in some embodiments. Some embodiments may obviate the need for extra heaters to tune the filter's operating bandwidth, as the same heaters used to tune the filter frequency can be used to tune filter bandwidth. In some embodiments, the device can be used as an add/drop filter.

As used herein, “filter” shall refer to an optical filter. The terms Mach-Zehnder Interferometer and MZI are used interchangeably to refer to an optical interferometer used to control or modify optical signal transmission.

“Microring resonator” or “MRR” as used herein refers to an optical ring resonator: an optical waveguide in the form of a ring. When an MRR is said to be “operably coupled” to an optical waveguide path (such as an arm of a MZI), this means that a portion of the light passing through the optical waveguide path will become coupled to the microring waveguide, passing around the circumference of the ring and thereby delaying the optical signal and introducing interference patterns into the waveguide that depend on the circumference of the microring and therefore the duration of the delay. The circumference of the microring will thus determine a frequency-dependent phase imparted to the optical signal passing through the waveguide path to which the microring is coupled. Optical frequencies near the resonance frequency of the MRR will acquire the frequency-dependent phase of the MRR, whereas optical frequencies far from this resonance frequency will not acquire an additional phase shift. The microring resonator may therefore act as a phase filter applied to the other optical waveguide path. As the resonance frequency of an MRR is determined by the amount of phase delay (also called “frequency-dependent phase”, “phase detune”, or simply “phase” in reference to resonators) introduced into the portion of the signal coupled to the ring, varying the phase of an MRR also means varying the resonance frequency and vice versa.

As used herein, the term “equal” may encompass both strict equality and substantial equality. The term “operably coupled” shall refer to two or more elements that act in concert or otherwise affect each other's operation, regardless of whether they are directly or indirectly physically connected or are not physically connected at all. The term “coupling coefficient” shall have its commonly known meaning in the field of resonators. In a simplified understanding, the coupling coefficient characterizes the interaction of two resonators at a resonance frequency.

In some aspects, the present disclosure describes a device. The device has an optical interferometer configured to split an optical input into a first path defined by a first arm and a second path defined by a second arm, a first-arm microring resonator (MRR) operably coupled to the first arm, a second-arm MRR operably coupled to the second arm, a first-arm MRR tuner and a second-arm MRR tuner. The first-arm MRR tuner and second-arm MRR tuner are configured to tune a frequency-dependent phase of the first-arm MRR and a frequency-dependent phase of the second-arm MRR, respectively, such that the first-arm MRR and second-arm MRR share a common coupling coefficient, and the first-arm MRR has a phase equal to the negative of a phase of the second-arm MRR. The device has a coupler configured to combine an optical output of the first path and an optical output of the second path to generate a filtered optical output having a filter bandwidth and filter center frequency based on the frequency-dependent phase of the first-arm MRR and the frequency-dependent phase of the second-arm MRR.

In some examples, the optical interferometer is a Mach-Zehnder Interferometer (MZI).

In some examples, the device further comprises at least one additional pair of tunable MRRs. Each pair of tunable MRRs comprises an additional first-arm MRR operably coupled to the first arm in series with the first-arm MRR, an additional second-arm MRR operably coupled to the second arm in series with the second-arm MRR, an additional first-arm MRR tuner and an additional second-arm MRR tuner. The additional first-arm MRR tuner and an additional second-arm MRR tuner are configured to tune a frequency-dependent phase of the additional first-arm MRR and a frequency-dependent phase of the additional second-arm MRR, respectively, such that the additional first-arm MRR and additional second-arm MRR share a common coupling coefficient, and the additional first-arm MRR has a phase equal to the negative of a phase of the additional second-arm MRR.

In some examples, the first-arm MRR tuner and second-arm MRR tuner each comprises a thermal tuner.

In some examples, each thermal tuner comprises a heater operably coupled to its respective MRR.

In some examples, each tuner comprises a thermal tuner operably coupled to its respective MRR.

In some examples, the frequency-dependent phase of the first-arm MRR and the frequency-dependent phase of the second-arm MRR may be tuned such that the spectral response of the filtered optical output exhibits a second-order transfer function, and the filter bandwidth may be adjusted between a minimum bandwidth and a maximum bandwidth without incurring insertion loss in the filtered optical output as a result of said bandwidth adjustment.

In some examples, the at least one additional pair of tunable MRRs comprises one additional pair of tunable MRRs. The frequency-dependent phase of each of the first-arm MRR, the second-arm MRR, the additional first-arm MRR, and the additional second-arm MRR may be tuned such that the spectral response of the filtered optical output exhibits a fourth-order transfer function, the filter bandwidth may be adjusted between a minimum bandwidth and a maximum bandwidth without incurring insertion loss in the filtered optical output as a result of said bandwidth adjustment, and the filtered optical signal has infinite attenuation at two out-of-band frequencies.

In some examples, the at least one additional pair of tunable MRRs comprises two additional pairs of tunable MRRs. The frequency-dependent phase of each of the first-arm MRR, the second-arm MRR, each additional first-arm MRR, and each additional second-arm MRR may be tuned such that the spectral response of the filtered optical output exhibits a eighth-order transfer function, the filter bandwidth may be adjusted between a minimum bandwidth and a maximum bandwidth without incurring insertion loss in the filtered optical output as a result of said bandwidth adjustment, and the filtered optical signal has infinite attenuation at six out-of-band frequencies.

In some aspects, the present disclosure describes a device. The device has an optical interferometer configured to split an optical input into a first path defined by a first arm and a second path defined by a second arm. The device has a first-arm microring resonator (MRR) operably coupled to the first arm. The device has a second-arm MRR operably coupled to the second arm. The device has a first-arm MRR tuner and a second-arm MRR tuner configured to tune a frequency-dependent phase of the first-arm MRR and a frequency-dependent phase of the second-arm MRR, respectively.

In some examples, the optical interferometer is a Mach-Zehnder Interferometer (MZI).

In some examples, the device further comprises a coupler configured to combine an optical output of the first path and an optical output of the second path to generate a filtered optical output.

In some examples, the device further comprises at least one additional pair of tunable MRRs. Each pair of tunable MRRs comprises an additional first-arm MRR operably coupled to the first arm in series with the first-arm MRR, an additional second-arm MRR operably coupled to the second arm in series with the second-arm MRR, and an additional first-arm MRR tuner and an additional second-arm MRR tuner configured to tune a frequency-dependent phase of the additional first-arm MRR and a frequency-dependent phase of the additional second-arm MRR, respectively.

In some examples, the first-arm MRR tuner and second-arm MRR tuner each comprises a thermal tuner.

In some examples, each thermal tuner comprises a heater operably coupled to its respective MRR.

In some examples, each tuner comprises a thermal tuner operably coupled to its respective MRR.

In some aspects, the present disclosure describes a method. An optical interferometer is used to split an optical input into a first path defined by a first arm and a second path defined by a second arm. A first-arm microring resonator (MRR) operably coupled to the first arm is used to apply a first frequency-dependent phase to the first path. A second-arm MRR operably coupled to the second arm is used to apply a second frequency-dependent phase to the second path. A first-arm MRR tuner and a second-arm MRR tuner are used to tune the first frequency-dependent phase and the second frequency-dependent phase, respectively, such that the first-arm MRR and second-arm MRR share a common coupling coefficient, and the first frequency-dependent phase is equal to the negative of the second frequency-dependent phase. A coupler is used to combine an optical output of the first path and an optical output of the second path to generate a filtered optical output having a filter bandwidth and filter center frequency based on the frequency-dependent phase of the first-arm MRR and the frequency-dependent phase of the second-arm MRR.

In some examples, the optical interferometer is a Mach-Zehnder Interferometer (MZI).

In some examples, an additional first-arm MRR operably coupled to the first arm in series with the first-arm MRR is used to apply a first additional frequency-dependent phase to the first path. An additional second-arm MRR operably coupled to the second arm in series with the second-arm MRR is used to apply a second additional frequency-dependent phase to the second path. An additional first-arm MRR tuner and an additional second-arm MRR tuner are used to tune the first additional frequency-dependent phase and the second additional frequency-dependent phase, respectively, such that the additional first-arm MRR and additional second-arm MRR share a common coupling coefficient, and the first additional frequency-dependent phase is equal to the negative of the second additional frequency-dependent phase.

In some examples, the first frequency-dependent phase and the second frequency-dependent phase may be tuned such that the filter bandwidth may be adjusted between a minimum bandwidth and a maximum bandwidth without incurring insertion loss in the filtered optical output as a result of said bandwidth adjustment.

