Photonic integrated circuit

A integrated optical circuit comprises an interferometer having a first optical path and a second optical path configured for regenerating an input signal entering the first path by interference at a first coupler between continuous wave (CW) signals from the two optical paths, and a third optical path configured such that a canceling signal passing therethrough cancels, at a second coupler, a traveling signal from the first arm. When the device is operated in a counter-propagative mode, the traveling signal is the CW signal from the first arm. When the device is operated in a co-propagative mode, the traveling signal is the input signal from the first arm.

BACKGROUND OF THE INVENTION

The present invention relates to all-optical signal regeneration and reshaping techniques, and more particularly, to a photonic integrated subcircuit having a honeycomb architecture for performing photonic signal regeneration and re-shaping (P2R) in the counter-propagative or co-propagative operation. The present invention further relates to an integrated optical circuit comprising multiple such subcircuits for performing multi-channel P2R.

P2R is an important function that lends itself to photonic integration. Photonic integrated circuits (PICs) realize several functions such as amplification, splitting, combining, filtering, and grooming on a single chip, and are key enablers of cheap and efficient network operation. All-optical regeneration and reshaping overcomes many limitations of electrical or optoelectronic counterparts, such as limitations on data rate, cost, flexibility, footprint, power consumption, etc.FIGS. 1aand1billustrate using a Mach-Zehnder (MZ) interferometer to perform P2R in counter-propagative and co-propagative operation modes, respectively.

FIG. 1ashows an co-propagative operation mode in which an input signal S from an arm33entering an arm32of a MZ interferometer at a coupler23is regenerated as an output at a coupler21by interference at coupler21between a continuous wave (CW) signal from arm34traveling through two optical paths or arms31and32of the interferometer. Each arm31,32is coupled to a semiconductor optical amplifier (SOA)12,13respectively, which works as a phase shifter as well as a signal amplifier. The input signal S interacts with the CW signal in arm32and causes it to change phase. Due to the well-known Cross-phase modulation (XPM) effect, the CW signal in arm32has a phase difference when the input signal S is in its high than when the input signal S is in its low. This phase difference induced by XPM is tuned to be 180 degrees, e.g., by tuning drive currents in the SOAs. The phase of the CW signal from arm31remains the same at the coupler21, irrespective of whether the input signal S is in its high or low state.

In addition, the drive currents of SOAs12,13in the arms31,32are tuned such that, when the input signal is in its low, the CW signals from the two arms31,32are 180 degrees out of phase at the coupler21. Thus, when the input signal is low, the CW signals from the two arms31,32destructively interfere with each other and generate a “0” output at the coupler21, and when the input signal is in its high, the phase of the CW signal in the arm32is flipped by the input signal S, thus constructively interferes at the coupler21with the CW signal from the arm31and generates an high output. If the amplitude of the CW signals out of arms31,32is “A”, the amplitude of the overall output generated at the coupler21will be4A2. Thus, the input signal S is regenerated as the overall output at the coupler21by interference between the CW signals from the arms31,32.

However, because the input signal S, which is modulated in the CW signal in arm32, also reaches the output arm35through the arm32, it represents a source of noise and would need to be filtered out. This would not be possible if the input signal S and the CW signal input are at the same wavelength. Therefore, a wavelength conversion has to be performed in co-propagation based devices.

FIG. 1bshows a circuit operates in a counter-propagative mode. As illustrated inFIG. 1b, a CW signal from arm35enters an MZ interferometer at the coupler21, thus the CW signal in the arm32travels in a direction opposite to that of the input signal S traveling in arm32. Similar to the co-propagative mode, the drive currents of SOAs12,13in the arms31,32are tuned such that, when the input signal is in its low, the CW signals from the two arms31,32have 180 degree out of phase at the coupler22. Thus, when the input signal S is in its low, the CW signals from the two arms31,32destructively interfere with each other and generates a “0” output at the coupler22, and when the input signal is in its high, the phase of the CW signal in the arm32is flipped by the input signal S, thus constructively interferes at the coupler22with the CW signal from the arm31and generates an high output. Therefore, the input signal S is regenerated as the overall output at the coupler22by interference between the CW signals from the arms31,32.

For the device to function effectively as an amplifier, it needs to support weak input signals. However, to flip the phase of the CW signal in the arm32when the input signal S is in its high, the input signal must be strong enough, i.e., approximately equal in intensity to the CW signal in arm32. To this end, a preamplifier SOA15is provided to preamplify the input signal S before it enters the arm31at the coupler23. Therefore, the SOA15in arm33needs to be effective in amplifying a weak input signal S. However, the CW signal in the arm32also reaches SOA15via arm33through the coupler23. Since the CW signal in arm32is typically much stronger than the input signal S, it saturates the SOA15. Therefore, SOA15cannot pre-amplify the weak input signal.

Therefore, there is a need for an improved photonic integrated circuit that can overcome the above shortcomings in the prior art.

SUMMARY OF THE INVENTION

The present invention provides a photonic integrated circuit which comprises an interferometer having a first optical path and a second optical path configured for regenerating an input signal entering the first optical path by interference at a first coupler between continuous wave (CW) signals from the two optical paths, and means for canceling, at a second coupler, a traveling signal from the first optical path. Preferably, the means for canceling comprises a third optical path configured such that a canceling signal passing therethrough and the traveling signal from the first optical path meeting at the second coupler have a same amplitude and a destructive phase difference so that they cancel each other at the second coupler.

In a preferred embodiment, the interferometer operates in a counter-propagative mode, and the traveling signal to be canceled is the CW signal passing through the first path. Thus, the CW signal from the first path will not reach a preamplifier for amplifying the input signal before it enters the first path.

In another preferred embodiment, the interferometer operates in a co-propagative mode, and the traveling signal to be canceled is an input signal passing through the first path. Thus, the input signal modulated with the CW signal in the first path can be cancelled before it reaches the first coupler. Therefore the output at the first coupler does not have a noise from the input signal. No filter for filtering the noise caused by the input signal is needed, and the device can work in a co-propagative operation mode even when the input signal and the CW signal have the same wavelength.

Preferably, the first optical path and the second optical path form the two arms of a first MZ interferometer, and the first optical path and the third optical path form the two arms of a second MZ interferometer.

Preferably, each of the first path and the third path is coupled to a respective SOA operated under saturation conditions, and drive currents of the SOAs are tuned to realize a destructive phase difference, at the second coupler, between the canceling signal from the third path and the traveling signal from the first path.

Preferably, the canceling signal and the traveling signal come from the same source signal by splitting the source signal at a coupler connecting the first and third optical paths. Preferably, when the device operates in a counter-propagative mode, the canceling signal and the traveling signal come from the same CW signal input; when the device operates in a co-propagative mode, the canceling signal and the traveling signal come from the same original input signal.

In a preferred embodiment, two or more photonic integrated subcircuits are incorporated to form a single multi-channel circuit. Preferably, alternative subcircuits are operated in co and counter propagative modes.

DETAILED EXPLANATION OF PREFERRED EMBODIMENTS

The present invention will be described in detail below with the preferred embodiments, in which similar reference numbers designate similar elements throughout the drawings.

FIG. 2illustrates a photonic integrated circuit of a preferred embodiment according to the teaching of the present invention, which is operated in a counter-propagative mode. In particular, a MZ interferometer41comprises two arms31and32, each coupled with an SOA12,13respectively. An input signal S enters the arm32at a coupler23and travels downward along the arm32, while continuous wave (CW) signals travel upwards through both the arms31and32of the MZ interferometer41. The CW signals traveling through the arms31and32meet and interfere with each other at the coupler22to generate an output representing the input signal S, as in the prior art shown inFIG. 1a.

Preferably, an SOA15is provided to amplify the input signal S so that a weak input signal can be amplified to be strong enough to flip the phase of the CW signal in the arm32.

According to the teaching of the present invention, the CW signal from the arm32is cancelled before it arrives the preamplifier SOA15. In the preferred embodiment as illustrated inFIG. 2, an optical path or arm37is provided to be connected with the arm32by couplers24and25at opposite ends, forming another MZ interferometer42. The circuit is preferably of a “honeycomb” architecture as illustrated. The input signal S entering the arm32at the coupler23is provided through the coupler24and is amplified by SOA15before it reaches the coupler24. Preferably, the arm37is coupled with an SOA16.

According to the teaching of the present invention, a CW signal travels upwards through the arm37to meet with, at the coupler24, the CW signal from arm32traveling upwards through the coupler23and the arm33. Drive currents in the SOAs16and13are tuned such that the CW signal from the arm37and the CW signal from the arm32meeting at the coupler24have the same amplitude and a destructive phase difference (preferably −180 degree) so that they cancel each other at the coupler24due to destructive interference. Thus, no CW signal reaches SOA15, and therefore SOA15can effectively amplify the input signal S so that it can be strong enough to flip the phase of the CW signal in arm32when the input signal is in its high.

Preferably, SOAs16and13are operated under saturation conditions, so that tuning the drive currents of them only changes their phases. Preferably, SOAs16and13are identical.

Preferably, the CW signals traveling through arms37,32and31are from the same CW signal input. In particular, as illustrated inFIG. 2, a CW signal input is divided at coupler27between arms35and38and then enters the arms31,32and37of the two MZ interferometers41and42at couplers21,26and25respectively. It is noted that the section between the couplers21and26of the MZ interferometer41is broken, so that the power entering arms31and32of the interferometer41are equal, and advantageously there is no interference between the CW signals while entering arm32.

In a preferred embodiment, all the couplers21–27illustrated inFIG. 2are 50/50 Multi-mode Interference (MMI) couplers. However, it is to be understood that they can be designed with other splitting ratios.

Preferably, all the three SOAs12,13and16in the three arms31,32and37are identical. However, they may be not identical, depending on intended applications. Alternatively, they may be replaced by other types of phase shifters.

Preferably, an SOA11is provided to pre-amplify the CW signal input before it arrives at the coupler27.

Preferably, an SOA14is provided to amplify the output at the coupler22.

FIG. 3illustrates a photonic integrated circuit of honeycomb architecture of another preferred embodiment according to the teaching of the present invention, which is operated in a co-propagative mode. In particular, a MZ interferometer41comprises two arms31and32, each coupled with an SOA12,13respectively. An input signal S enters the arm32at a coupler23and travels downward along the arm32, and continuous wave (CW) signals also travel downwards through the arms31and32of the MZ interferometer41. The CW signals traveling through the arms31and32meet and interfere with each other at the coupler27to generate an output representing the input signal S, as in the prior art shown inFIG. 1b. It is noted that the section between the couplers21and26in the MZ interferometer41is broken.

Preferably, an SOA15is provided to amplify the input signal S so that a weak input signal will be amplified to be strong enough to flip the phase of the CW signal in the arm32.

According to the teaching of the present invention, the input signal from the arm32is cancelled before it arrives at the coupler27. In the preferred embodiment as illustrated inFIG. 3, an optical path or arm37is provided to be connected with the arm32by couplers24and25at opposite ends, forming another MZ interferometer42. Preferably, an SOA16is coupled with arm37.

According to the teaching of the present invention, a canceling signal travels downwards through the arm37to meet with, at the coupler25, the input signal (which is modulated with the CW signal) in arm32traveling downwards through the the arm32. Drive currents in the SOAs16and13are tuned such that the canceling signal from the arm37and the input signal from the arm32meeting at the coupler25have the same amplitude and a destructive phase difference so that they cancel each other at the coupler25. Thus, no input signal reaches coupler27, and therefore the overall output generated at the coupler27does not have a noise from the input signal. No filter for filtering the input signal is needed, and therefore the device can work in the co-propagative mode even when the input signal and the CW signal have the same wavelength.

Preferably, SOAs16and13are operated under saturation conditions so that tuning the drive currents of them only changes their phases. Preferably, SOAs16and13are identical.

Preferably, the canceling signal traveling through the arm37and the input signal traveling through the arm32are from the same original input signal S. In particular, as illustrated inFIG. 3, the original input signal S input is divided at coupler24between arms32and37after it is preamplifies by SOA15.

In a simplified embodiment, all the couplers22–27illustrated inFIG. 3are 50/50 Multi-mode Interference (MMI) couplers. However, it is to be understood that they can be designed with other splitting ratios.

Preferably, all the three SOAs12,13and16in the three arms31,32and37are identical. However, they may be not identical, depending on intended applications. Alternatively, they may be replaced by other types of phase shifters.

Preferably, an SOA14is provided to preamplify the CW signal input before it enters the MZ interferometer41at the coupler27.

Preferably, an SOA11is provided to amplify the output at the coupler27.

Preferably, a multi-channel optical circuit may incorporate multiple subcircuits. Preferably, at least some of the subcircuits are those according to the present invention as described above. Preferably, the subcircuits are arranged such that alternative subcircuits are operated in co and counter propagative modes, respectively. Preferably, adjoining subcircuits share a common optical path.

As a preferred embodiment,FIG. 4shows a multi-channel arrangement comprising four subcircuits. The two subcircuits associated with signals1and3have a “honeycomb” architecture and are operated in the counter-propagative mode (as illustrated inFIG. 2), and the two subcircuits associated with signals2and4use conventional co-propagation based WL conversion. Overall, the multi-channel circuit is more compact than isolated single channel circuits that implement the same functionality. This is because a single arm of the design wears different hats and can be a part of different interferometers.

The above have described in detail the preferred embodiments of the present invention. However, it should be appreciated that without departing the gist of the present invention, numerous adaptations, variations and modifications are possible to a person skilled in the art. For example, with proper designs, the couplers and SOAs in the embodiments do not have to be identical, and the broken section between the couplers21and26may be connected. Therefore, the scope of the present invention is intended to be solely defined in the claims.