BACKGROUND OF THE DISCLOSURE

Field of the Disclosure

[001] The disclosure relates generally to optical communications systems, and more particularly to bidirectional coherent transmission of optical signals via a single optical fiber.

Background of the Disclosure

[002] Various fiber optic communication systems have been developed for conveying information bidirectionally or reciprocally, i.e. symmetrically with respect to the direction of propagation, between first and second locations. There are two fundamental topologies to achieve bidirectional coherent transport: dual-fiber and single-fiber. The dual fiber configuration requiring two strands of fibers - one for transmission and one for receiving signals at the same or different wavelengths - has been a long-time established industrial standard.

[003] The main motivation for considering bidirectional transmission over a single optical fiber instead of two unidirectional fibers is the reduction of the infrastructure (fibers, optical splitters and optical amplifiers) by a factor two and the potential cost reduction by an integrated transceiver design. Also, a single fiber configuration is less susceptible to connection errors because there are fewer connections or end points in the network. In addition, customer can also choose to use single fiber to decrease redundancy in the network.

[004] In recent years, the mainstream single-fiber transmission technology is based on two different wavelengths traveling in opposite directions (BiDi transmission) which doubles the fiber capacity. The key to this technology is wavelength division multiplexing/de-multiplexing (BiDi WDM) couplers, also known as diplexers, which combine and separate data transmitted over a single fiber based on the wavelengths of the light. Generally, this WDM coupler is integrated into a standard interface photonic optical transceiver module. The WDM however is a sophisticated and thus expensive device.

[005] FIG. 1 illustrates a bidirectional transmission system using a single optical fiber to send data in both directions Tx, Rx. Two BiDi WDM transceivers each are used to combine and separate wavelengths. If, for example, one of BiDi WDM transceivers has a receiving wavelength of 1550 nm and a transmit wavelength of 1310 run, then it is necessary to use its matching module which has a receiving wavelength of 1310 mn and a transmit wavelength of 1550 nm. As a consequence, the capacity of the single fiber is twice the capacity of the two fiber system. But does it meet the necessary' requirements imposed by a modem web-scale world? On-demand content, bandwidth-hungry mobile apps, high-definition video streaming, and new' cloud- based IT applications are driving massive scale and unpredictable traffic patterns. Network capacities are increasing by 25 to 50 percent every year, and current systems just cannot keep up with this rapid scalability.

[006] Coherent optics at least partially addresses the capacity problems network providers are facing, it takes the typical ones and zeroes in a digital signal — the blinking on and off of the light in the fiber ^. and uses sophisticated technology to modulate the amplitude and phase of that light and send the signal across each of two polarizations. This, in turn, imparts considerably more information onto the light propagating through a fiber optic cable. Thus the BiDi WDM coherent detection is associated with at least: 1. high order amplitude/phase modulation; 2. polarization multiplexing; 3. coherent detection using a local oscillator in the receiver; and 4. high-speed analog-digital converters (ADCs) and sophisticated digital signal processing in the receiver.

[007] There are very few practical implementations of coherent technologies on the market today. Mainly it can be explained by the complexity of these systems requiring an unusual mixture of optical, digital and radio modem experience, which is coupled with a very demanding optical transmitter and receiver circuit design.

[008] A need therefore exists for a cost-effective bidirectional single-fiber coherent transmission system.

SUMMARY OF THE DISCLOSURE

[009] The disclosed cost-effective Bi-Di single-fiber coherent transmission system increases the fiber capacity by providing two single frequency (SF) continuous wave (CW) lasers operating at respective, different wavelengths in each of the modules. Due to WDM technology, only signals at respective different wavelengths are simultaneously guided through a single fiber in opposite directions providing the interference-free transmission of these signals between two modules. [010] The modules of the disclosed bidirectional coherent system have the same configuration and include respective photonic Bi-Di WDM transceivers. The transceivers each are integrated in a silicon photonic chip, which is configured with optoelectronic devices on a photonically- enabied integrated circuit (PIC), and include signal transmitting and signal receiving branches. The transmitting branch is configured with one or multiple phase and polarization optical modulators, whereas the receiving branch has a photodiode (PD). In accordance with one feature of the disclosure, the lasers of each module generate respective reference signals at different wavelengths both of which are coupled into the transmitting and receiving branches of respective transceivers.

[011] The signal transmitting branches each are configured with one or more modulators receiving reference signals and modulating them so as to output respective modulated signals In the disclosed schematic, the phase and polarization, are utilized to carry the data in opposite directions between the transceivers.

[012] The Bi-Di transceivers are fitted with Wavelength Division Multiplexing (WDM) couplers which, in general, combine and separate data transmitted over a single fiber based on the wavelengths of the light. In particular, the WDMs each receive modulated signals at respective wavelengths and sort out one of them such that only modulated signals at respective different wavelengths are transmitted in opposite directions through the single fiber linking the WDM couplers.

[013] The transmitted modulated signals are received by respective WDMs of the transceivers which guide the received modulated signals to respective receiving branches. As known, a PD cannot directly detect the phase coding. To provide the decoding of the received data in the disclosed system each of the transmitted modulated signals interferes with both reference signals which results in two beat frequencies.

[014] In accordance with one of salient features of the disclosure, only the beating of reference and modulated signals having the same wavelength provide the phase information that will be converted from the optical domain to the electrical domain by a PD in each of the receiving branches. The beat frequency generated by the interference of reference and modulated signals at respective different wavelengths lies outside the responsivity range of the PD and is practically undetectable by the PD. As known, the responsivity of a PD beyond a 0 - 45 G Hz frequency range drops sharply. Thus, when the reference signal at wavelength in one of the transceivers interferes with modulated signal and reference signal and modulated signal beat against each other in the other transceiver, the respective resulting beat frequencies each lie in a spectral range outside the detection bandwidth of the PD. As a result, the disclosed PDs each function as a spectral filter.

[015] Other features of the disclosed system include a few structural additions to the above disclosed inventive concept. Thus, for instance, WDM couplers may be integrated in respective PICs or may be pigtailed. The same modification relates to each pair of laser diodes which are this may be pigtailed or fully integrated in the PIC. The modules each can include additional photonic devices well known to one of ordinary skill in the telecommunication art and typically integrated in the PIC.

BRIEF DESCRIPTION OF THE DRAWINGS

[016] The above and additional features of the disclosed structure will be discussed in detail below and illustrated by the following drawings, in which:

[017] FIG. 1 is a simplified optical schematic of the known single fiber bidirectional communication system;

[018] FIG. 2 is the inventive schematic including two terminals or modules;

[019] FIG. 3 is the schematic of the disclosed structure of FIG. 2 illustrating a multi-terminal - hub communication system.

[020] FIG. 4 illustrates the results of optical signal to noise ratio (OSNR) vs. bit error rate (BER) observed in an experimental system configured in accordance with the present disclosure. [021] FIG. 5 illustrates the sensitivity test of the experimental system of FIG. 4.

SPECIFIC STRUCTURE

[022] Disclosed herein are architectures for a bidirectional coherent transmission system based on the use of photonic devices. The disclosed architectures offer several advantages including, among others, a high transmission capacity fiber and simple cost efficient configuration.

[023] Referring to FIG. 2, disclosed system 10 thus includes at least one pair of spaced apart modules 12, 14 in optical communication with one another. The modules 12, 14 each include a pair of SF diode lasers 16, 18 packaged with a PIC 20. The SF diode lasers 16, 18 each are pigtailed, i.e., the optical connection between the SF lasers and PIC 20 is realized by respective single mode (SM) fibers 22, 24 both coupled into a 50-50 coupler 36. Alteratively, the SF lasers may be integrated in PIC. 20. Each module is configured to send/receive modulated signals at respective different wavelengths and compound them to one optical fiber without interference by using WDM technology. In the latter, communications flow continuously in one direction at one wavelength and simultaneously in the other direction at the other wavelength.

[024] The PICs 20 each have integrated high speed phase modulator 25 operative to modulate the phase of reference signals at respective wavelengths and output respective modulated signals which further propagate through a bidirectional WDM 26, 28 wherein one of the modulated signals is filtered out. The remaining modulated signal high transmitted by modulator 25 of module 12 and modulated signal high transmitted by other modulator 25 of module 14 are coupled into respective ends of a SM fiber 30 and guided in the opposite directions. The transmitted modulated signals are coupled into respective WDM couplers 28, 26 which further transmit these signals to respective receivers 32 of modules 14, 12.

[025] It is well known that the phase coding on the light-wave cannot be detected directly by the photodetector (PD) of the receiving branch. Therefore, when the received modulated signal, for example beats against reference signals in the receiving branch 32 of module 14, only one beat frequency, which is generated by beating the reference and modulated signals at the same wavelength is detected by PD 34. The other beat frequency produced as a result of interference between modulated signal and reference signal - signals at different wavelengths - is beyond the detection range of PD 34. Similarly, PD 34 integrated in other module 12 detects the beat frequency between transported modulated signal at and reference signal at the same wavelength while the interference between the signals at different wavelengths produces a weak constant current amounting to insignificant background noise. The signals detected by respective PDs 34 are further processed in a digital signal processor (DSP), as customary in the art of communication.

[026] FIG. 3 illustrates inventive system 100 transmitting data between a plurality of individual spaced modules 12l...l2n and a hub 46 having respective adjacent modules 14l ... 14n which are paired with respective modules 12l - 12n. As a result, modules 12l ... 12n and modules 14i ... 14n form respective two clusters of modules. [027] The modules 12i.,,12n each have the configuration identical to the configuration of module 12 of FIG. 2. Thus each of modules 12l... 12n includes a pair of SF lasers 16], 1 Si ... 16 _n 18 _n , PIC 20 with integrated modulator 25 and receiver 32, WDM 42, and PD 34. The lasers output respective reference signals at wavelengths which are all different from one another. Thus, reference signals at respective wavelengths of each individual module 12 differ from each other, and reference signals at respective wavelengths of modules 12i and 12n also differ from one another. After filtering modulated signals at respective wavelengths by respective WDMs 42, the remaining modulated signals of respective modules 12l - 12n are guided through respective channels of an output WDM 44, which is coupled to all individual WDMS 25. The transmitted modulated signals further propagate in a single SM fiber 50 towards WDM 48 ofhub 46 which further transmits these signals to respective receiving branches 34 of modules 14i ...14n via individual WDMs 42. The number of modules and channels of WDM 44 are limited only by practical considerations.

[028] The WDM 48 is a single WDM in hub 46, i.e,, individual modules 14i .. _. 14n don’t have respecti ve WDMs which can be explained by limited space of hub 46 and close proximity of modules 14 to WDM 48. In contrast, WDMs 25 of respective modules 12 can be spaced from output WDM 44 at tens of kilometers. Returning to WDM 48, it is easy to see that this WDM is common to all modules 14l - 14n of hub 46. Functionally, upon filtering output modulated signals at respective wavelengths common WDM 48 transmits modulated signals at respective wavelengths to output WDM 44 via fiber 50 in the direction opposite to that of signals Decoding of the received modulated signals is done by respective DSPs in the manner disclosed above.

[029] The inventive system 10 ofFIG. 2 was tested to investigate the system’s penalty inlcuding optical signal to noise ratio (OSNR) and sensibility of 100G signal as compared to an analogous two fiber communication system. The simulation parameteres of the tested system include a 100G transmission rate, 500 km transmission distance, 13 dbm power of lasers, 200 ghz lasers frequency separation, frequency offset (tested channel) - 1.8GHz. FIG. 4 illustrates the bit error rate as a function of the OSNR. As can be seen from the fgure no degradation was observed. FIG. 5 illustrates the results of the sensibility test. As shown at OSNR of 14 dB a 2 dB penalty was observed. [030] It is to be appreciated that various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the scope of the disclosure. Accordingly, the foregoing description and drawings are by way of example only.