BIDIRECTIONAL SINGLE-FIBER COHERENT TRANSMISSION SYSTEM

Abstract

The Bi-Di coherent transmission system is configured with at least one pair of modules coupled to one another via a single fiber. The modules each are configured with a pair of laser outputting two reference signals at respective different wavelengths λ.sub.1o and λ.sub.2o, photonic transceiver and a wavelength division multiplexer (WDM) coupler. The photonic transceivers each have transmitter and receiver branches integrated in a photonic circuit and receiving the reference signals. The transmitter is configured to modulate the received reference signals λ.sub.1oT and λ.sub.2oT which are further coupled into the WDM coupler. The WDM couplers each sort out one of the modulated signals and transmit the other modulated signal such that the transmitted modulated signal at different wavelengths λ.sub.1oT and λ.sub.2oT are coupled into respective opposite ends of the fiber and propagate towards one another in opposite directions. The transmitted modulated signals arc coupled into respective branches through the WDM couplers with each transmitted modulated signal interfering with the reference signals at wavelengths λ.sub.1oT and λ.sub.2oT. The photodiodes of respective receiving brandies are configured to detect a beat frequency of the interfering signals at the same wavelength.

Claims

1. A bidirectional (BiDi) coherent transmission system comprising: at least one pair of spaced modules each configured with: two lasers outputting respective reference signals at different wavelengths λ.sub.1o and λ.sub.2o, a photonic transceiver including transmitting and receiving branches which both receive the reference signals, the transmitting branches of respective transceivers each being configured to output respective modulated signals λ.sub.1oT and λ.sub.2oT, a Bi-Di wavelength division multiplexer (WDM) coupler receiving modulated signals λ.sub.1oT and λ.sub.2oT and sorting out one of them, so that only modulated signals at respective different wavelengths λ.sub.1oT and λ.sub.2oT are transmitted by respective WDMs; and a single fiber guiding transmitted modulated signals λ.sub.1oT and λ.sub.2oT in respective directions opposite to one another towards respective receiving branches which each have a photodiode (PD), the transmitted modulated signals each interfering with the reference signals at respective wavelengths λ.sub.1o and λ.sub.2o such that each PD detects a beat frequency of the reference and transmitted modulated signals at the same wavelength but is not responsive to a beat frequency of the reference and transmitted modulated signals at respective different wavelengths.

2. The Bi-Di coherent transmission system of claim 1, wherein the transceivers each are integrated in a photonic integrated circuit (PIC).

3. The Bi-Di coherent transmission system of claim 2, wherein the lasers each are integrated in respective PICs or are pigtailed.

4. The Bi-Di coherent transmission system of claim 3, wherein the lasers each are a single frequency diode laser or single frequency fiber laser.

5. The Bi-Di coherent transmission system of claim 3 further comprising a pair of couplers each located between the pigtailed lasers and transceiver and configured to split an output of the lasers between the transmitting and receiving branches of each PIC.

6. The Bi-Di coherent transmission system of claim 2, wherein the transmitting branches each are configured with one or more phase modulators integrated in the PIC.

7. The Bi-Di coherent transmission system of claim 2, wherein the WDM couplers each are integrated in the PIC or pigtailed.

8. The Bi-Di coherent transmission system of claim 1, wherein the fiber is a single transverse mode fiber.

9. The Bi-Di coherent transmission system of claim 1 further comprising a plurality of additional modules grouped in one and other clusters such that each module of one cluster is paired with a corresponding module of the other cluster to form a plurality of pairs of modules.

10. The Bi-Di coherent transmission system of claim 9, wherein the modules of the other cluster are located in a hub and optically coupled to the WDM which is common to the modules of the hub, the modules of the one cluster having respective WDMs which are optically coupled to an output WDM, the output WDM and WDM common to the modules of the other cluster being coupled to respective opposite ends of the single fiber.

11. The Bi-Di coherent transmission system of claim 10, wherein the lasers of each pair of live modules output different reference signals at respective wavelengths which differ from wavelengths of respective reference signals of all other pairs.

12. The Bi-Di coherent transmission system of claim 9, wherein the output WDM and WDM of the other cluster each have a multichannel configuration.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The above and additional features of the disclosed structure will be discussed in detail below and illustrated by the following drawings, in which:

[0017] FIG. 1 is a simplified optical schematic of the known single fiber bidirectional communication system;

[0018] FIG. 2 is the inventive schematic including two terminals or modules;

[0019] FIG. 3 is the schematic of the disclosed structure of FIG. 2 illustrating a multi-terminal hub communication system.

[0020] 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.

[0021] FIG. 5 illustrates the sensitivity test of the experimental system of FIG. 4.

SPECIFIC STRUCTURE

[0022] 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.

[0023] 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. Alternatively, the SF lasers may be integrated in PIC 20. Each module is configured to send/receive modulated signals at respective different wavelengths λ.sub.1o and λ.sub.2o 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.

[0024] The PICs 20 each have integrated high speed phase modulator 25 operative to modulate the phase of reference signals at respective wavelengths λ.sub.1o and λ.sub.2o and output respective modulated signals λ.sub.1oT and λ.sub.2oT which further propagate through a bidirectional WDM 26, 28 wherein one of the modulated signals is filtered out. The remaining modulated signal high λ.sub.1oT transmitted by modulator 25 of module 12 and nodulated signal high λ.sub.2oT 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 λ.sub.1oT, λ.sub.2oT are coupled into respective WDM couplers 28, 26 which further transmit these signals to respective receivers 32 of modules 14, 12.

[0025] It is well known that the phase coding on the light-wave cannot be detected directly by the photodetcctor (PD) of the receiving branch. Therefore, when the received modulated signal, for example λ.sub.1oT, beats against reference signals λ.sub.1o and λ.sub.2o 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 λ.sub.1o, is detected by PD 34. The other beat frequency produced as a result of interference between modulated signal λ.sub.1oT and reference signal λ.sub.2o—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 λ.sub.2oT and reference signal at the same wavelength λ.sub.2o, 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.

[0026] FIG. 3 illustrates inventive system 100 transmitting data between a plurality of individual spaced modules 12.sub.1 . . . 12n and a hub 46 having respective adjacent modules 14.sub.1 . . . 14n which are paired with respective modules 12.sub.1-12n. As a result, modules 12.sub.i . . . 12n and modules 14.sub.1 . . . 14n form respective two clusters of modules.

[0027] The modules 12.sub.1 . . . 12n each have the configuration identical to the configuration of module 12 of FIG. 2. Thus each of modules 12.sub.1 . . . 12n includes a pair of SF lasers 16.sub.1, 18.sub.1 . . . 16.sub.n 18.sub.n, PIC 20 with integrated modulator 25 and receiver 32, WDM 42, and PD 34. The lasers output respective reference signals at wavelengths λ.sub.1o-λ.sub.1no . . . λ.sub.2o-λ.sub.2no which are all different from one another. Thus, reference signals at respective wavelengths λ.sub.1o and λ.sub.2o, (λ.sub.1no and λ.sub.2no) of each individual module 12 differ from each other, and reference signals at respective wavelengths λ.sub.1o and λ.sub.1no (and λ.sub.2no, λ.sub.2o) of modules 12.sub.1 and 12n also differ from one another. After filtering modulated signals at respective wavelengths λ.sub.2oT and λ.sub.2noT by respective WDMs 42, the remaining modulated signals λ.sub.1oT and λ.sub.1noT of respective modules 12.sub.1-12n 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 of hub 46 which further transmits these signals to respective receiving branches 34 of modules 14.sub.1 . . . 14n via individual WDMs 42. The number of modules and channels of WDM 44 are limited only by practical considerations.

[0028] The WDM 48 is a single WDM in hub 46, i.e., individual modules 14.sub.1 . . . 14n don't have respective 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 14.sub.1-14n of hub 46. Functionally, upon filtering output modulated signals at respective wavelengths λ.sub.1o-λ.sub.1no, common WDM 48 transmits modulated signals at respective wavelengths λ.sub.2o-λ.sub.2no to output WDM 44 via fiber 50 in the direction opposite to that of signals λ.sub.1oT . . . λ.sub.1noT. Decoding of the received modulated signals is done by respective DSPs in the manner disclosed above.

[0029] The inventive system 10 of FIG. 2 was tested to investigate the system's penalty including optical signal to noise ratio (OSNR) and sensibility of 100 G signal as compared to an analogous two fiber communication system. The simulation parameteres of the tested system include a 100 G transmission rate, 500 km transmission distance. 13 dbm power of lasers, 200 ghz lasers frequency separation, frequency offset (tested channel)—1.8 GHz. FIG. 4 illustrates the bit error rate as a function of the OSNR. As can be seen from the figure 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.

[0030] 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.