WO2015099294A1

WO2015099294A1 - Optical transmitter based on interferometric noise suppressed and pulsed bls

Info

Publication number: WO2015099294A1
Application number: PCT/KR2014/011215
Authority: WO
Inventors: Chang Hee Lee; Sang Hwa Yoo; Sang Rok Moon; Myeong Gyun Kye; Seung Chul Han; Dong Hyeop Son
Original assignee: Korea Advanced Institute Of Science And Technology
Priority date: 2013-12-24
Filing date: 2014-11-20
Publication date: 2015-07-02
Also published as: DE112014000187T5; KR20150074509A; KR101538914B1

Abstract

A pulsed-BLS-seeded optical transmitter based on interferometric noise suppression is disclosed. A light source unit according to an embodiment of the present disclosure includes a broadband light source (BLS); an injection unit configured to inject a pulse into an output of the BLS's first stage to output a pulsed light; an interferometric noise suppressing unit configured to: distribute the pulsed light into first and second outputs, delay the first output, and combine the second output and delayed first output; and a light amplifying unit configured to amplify output from the interferometric noise suppressing unit.

Description

OPTICAL TRANSMITTER BASED ON INTERFEROMETRIC NOISE SUPPRESSED AND PULSED BLS

The present disclosure relates to an optical transmitter based on interferometric noise suppressed and pulsed BLS.

Recently, in order to cope with the subscribers’ increasing demands for multimedia services, a wavelength division multiplexing (WDM) optical communication system is considered as a next-generation network, which is able to provide a very high bit-rate.

FIG. 1 is a view broadly illustrating an optical transmitter capable of operating regardless of wavelength in a general WDM system.

Here, the broadband light source (BLS) (100) is a light source based on amplified spontaneous emission (ASE). The reflective modulator (RM) (200) is an element which modulates and transmits signals spectrum-sliced by the wavelength division multiplexing/demultiplexing unit (300), which is also performing amplification, modulation, and noise suppression.

The structure of conventional BLS (100) will be described in the following. Fig. 2 is a detailed block diagram illustrating the BLS (100) illustrated in FIG. 1.

The BLS (100) is formed of a first stage and a second stage. An initial light of C-band or L-band generated by the pump laser diode (LD) (110) at the second stages is amplified by the erbium doped fiber (EDF)(20) generating an initial C-band or L-band ASE. And then the light of which gain is flattered by the gain flattering filter (GFF)(130) is amplified by the EDF (140) and the pump LD (150) to be finally outputted.

However, such conventional light source has problems that, since high-powered pump LDs(110,150) are required at each stage, the cost is increasing, and due to optical intensity noises, there occur limitations in implementing optical transmission systems having high bit-rates more than 10Gb/s.

The present disclosure is to provide a cost-effective and performance-improved optical transmitter, by injecting a pulsed-BLS of which interference noise is suppressed.

In one general aspect of the present disclosure, there is provided light source unit, comprising: a broadband light source (BLS); an injection unit configured to inject a pulse into an output of the BLS to output a pulsed light; an interferometric noise suppressing unit configured to: distribute the pulsed light into first and second outputs, delay the first output, and combine the second output and delayed first output; and a light amplifying unit configured to amplify output from the interferometric noise suppressing unit.

In another general aspect of the present disclosure, there may be provided an optical transmitter, comprising: the light source unit of claim1; a demultiplexing unit configured to demultiplex a pulse-seeded light inputted from the light source unit; a plurality of reflective modulators configured to modulate a pulse-seeded light received from the demultiplexing unit into an electrical NRZ signal; and a multiplexing unit configured to multiplex a plurality of signals inputted from the plurality of reflective modulators.

The exemplary embodiments of the present disclosure as described in the above have an advantageous effect in that the threshold (FECth) of forwards error correction (FEC) code can be satisfied upon a low seed power and the received power can be maintained identically even when the seed power is reduced.

FIG. 2 is a detailed block diagram illustrating the BLS (100) illustrated in FIG. 1.

FIG. 3 is a block diagram of an optical transmitter according to an exemplary embodiment of the present disclosure.

FIG. 4a is an exemplary view illustrating an output signal of a pulse injection unit illustrated in FIG. 3.

FIG. 4b is an exemplary view illustrating an output signal of a interferometric noise suppressing unit illustrated in FIG. 3.

FIG. 4c is an exemplary view illustrating an output signal of an optical transmitter according to an exemplary embodiment of the present disclosure.

FIGs. 5 and 6 are exemplary views for describing performance of a light source unit according to an exemplary embodiment of the present disclosure.

FIG. 7 is a block diagram illustrating an optical transmitter according to another exemplary embodiment of the present disclosure.

FIGs. 8 and 9 are exemplary embodiment block diagrams illustrating a WDM system according to the present disclosure.

FIGs. 10a and 10b are exemplary views illustrating bit-error rate(BER)s for describing 10Gb/s transmission characteristics by injection lights sources.

FIG. 11 is an exemplary view for comparing performances of pulsed-BLSs by injection light sources.

Various exemplary embodiments will be described more fully hereinafter with reference to the accompanying drawings, in which some exemplary embodiments are shown. The present inventive concept may, however, be embodied in many different forms and should not be construed as limited to the exemplary embodiments set forth herein. Rather, the described aspect is intended to embrace all such alterations, modifications, variations, and equivalents that fall within the scope and novel idea of the present disclosure.

Hereinafter, referring to enclosed figures, an exemplary embodiment of the present disclosure will be described in detail.

As illustrated in the figure, the optical transmitter (1) according to an exemplary embodiment of the present disclosure, may include a light source unit (10), a wavelength division multiplexing/demultiplexing unit (20), a reflective modulator (RM) (30), and a circulator (40). However, although the reflective modulator (RM) (30) is illustrated as formed of a single element in the figure, it is apparent that the RM (30) may be formed of a single element, also may be formed of a plurality of elements. An exemplary embodiment of the RM (30) formed of a plurality of elements will be described later.

According to an exemplary embodiment of the present disclosure, the light source unit (10) may be formed of a first stage, a second stage, and a third stage.

The first stage is to generate ASE of an initial C-band or L-band, and may include the BLS (11). The BLS (11) is a polarization BLS generating ASE of a C-band or L-band, which may be a cost-effective superluminescent diode (SLD) or a reflective semiconductor optical amplifier (RSOA). In addition, the BLS (11) may be a Fabry-Perot laser diode (F-P LD) or a mutually injected F-P LD, in order to reduce dispersion effect. However, the BLS (11) according to an exemplary embodiment of the present disclosure is not limited hereto. Therefore, various types of cost-effective elements capable of generating ASE light may be used.

The second is a stage for eliminating interference of pulse-injected light sources, and may include a pulse injection unit (12) and a interferometric noise suppressing unit (13). In addition, the pulse injection unit (12) may include a first polarization control unit (12A), a pulse providing unit (12B), and a modulating unit (12C). The interferometric noise suppressing unit (13) may include a distributing unit (13A), a second polarization control unit (13B), an optical delay (ODL) (13C), a third polarization control unit (13D), and a polarization beam combiner (PBC) (13E).

At first, the modulating unit (12C) is a modulator having a predetermined polarization, and may be, for example, a Mach-Zehnder modulator (MZM). Because the modulating unit (12C) has a predetermined polarization and the BLS (11) also has a predetermined polarization, the first polarization control unit (12A) may control polarizations of the BLS (11) and the modulating unit (12C) to be polarized in the same direction (for example, in X-direction).

The pulse providing unit (12B) may provide a clock signal having the identical frequency to the data rate provided to the reflective modulator (30). For example, the pulse providing unit (12B) may provide a clock rate having frequency of 10.7GHz.

That is, an ASE injection light outputted from the BLS (11) may be modulated into a pulse-BLS. FIG. 4a is an exemplary view illustrating an output signal of the pulse injection unit illustrated in FIG. 3. As illustrated in the figure, the light outputted from the pulse providing unit (12) is a pulse-applied continuous wave, which is single-polarized.

Meanwhile, the distributing unit (13A) of the interferometric noise suppressing unit (13) distributes the inputted pulse-BLS by 50:50 and then provide to the first path ① and the second path ②.

The light delaying unit (13C) of the second path ② may delay the inputted pulse-BLS by one pulse cycle.

The second polarization control unit (13B) and the third polarization control unit (13C) may respectively control the light of the first path ① and the light which has passed through the optical delay (13C) to be perpendicular to each other. That is, when the light which has passed through the second polarization control unit (13B) is polarized in X-direction, the light which has passed through the second polarization control unit (13D) may be polarized in Y-direction.

The PBC (13E) combines the lights of the first path ① and the second path ②, whereby the light in X-direction and the light delayed by one pulse cycle in Y-direction may be combined and outputted.

FIG. 4b is an exemplary view illustrating an output signal of the interferometric noise suppressing unit (13) illustrated in FIG. 3. As illustrated in the figure, the light in X-direction and the light delayed by one pulse cycle in Y-direction are being combined and outputted.

The third stage may include a pre-filter (14) and a light amplifying unit (15). The pre-filter is an optional alternative, which is for alleviating filtering effects of the wavelength division multiplexing/demultiplexing unit (20). Bandwidth of the pre-filter (14) may be around 70% of bandwidth of the wavelength division multiplexing/demultiplexing unit (20), and may spectrum-split the light inputted in a predetermined bandwidth.

The light amplifying unit (15) may be, for example, an erbium doped fiber amplifier (EDFA), which may amplify the inputted light and then output it.

Such the light outputted from the light source unit (10) may be inputted to the the wavelength division multiplexing/demultiplexing unit (20) by the circulator (40). Then the wavelength division multiplexing/demultiplexing unit (20) may demultiplex the light by spectrums and may input each of the demultiplexed lights to the modulator (30). The wavelength division multiplexing/demultiplexing unit (20) may be, for example, an arrayed-waveguide grating (AWG).

According to an exemplary embodiment of the present disclosure, the reflective modulator (30) modulates light demultiplexed by the wavelength division multiplexing/demultiplexing unit (20) with data, and reflects to input to the wavelength division multiplexing/demultiplexing unit (20) again. The wavelength division multiplexing/demultiplexing unit (20) may multiplex the demultiplexed and modulated light inputted from the reflective modulator (30). The multiplexed light may be transmitted to the transmission fiber (50) through the circulator (40).

According to an exemplary embodiment of the present disclosure, the electrical signal inputted to the reflective modulator (30) may be a non-return-to-zero (NRZ) signal. When an injection light of a pulse train is inputted the reflective modulator (30) driven by the NRZ signal, a return-to-zero (RZ) signal is outputted. In general, in respect to a coherent light, the RZ format is more vulnerable to optical fiber dispersion than the NRZ format, because the RZ format has broader spectrum linewidth than the NRZ format. However, in respect to a spectrum-splitted light, because the bandwidth of the spectrum linewidth is determined by division, the RZ format of which inter-bit-margin is larger has an advantage to broaden a dispersion-induced pulse over the NRZ format. Thus, the RZ format has a better tolerance to color dispersion. Moreover, the RZ signal has a better signal-to-noise ratio than the NRZ signal to enhance sensitivity of the transmitter.

In general, a modulator or a transmitter having a broad bandwidth is required in order to generate the RZ signal. However, according to an exemplary embodiment of the present disclosure, because the RZ signal may be generated using the electrical NRZ signal, it is possible to generate a signal in RZ format to enhance sensitivity of the transmitter, without a broadband modulator or transmitter.

Meanwhile, as illustrated in FIG. 3, an variable optical delay line (VODL) may be used at a front end of the reflective modulator (30). The VODL may synchronize the light pulse injected to the reflective modulator (30) and the modulated electric NRZ signal.

FIG. 4c is an exemplary view illustrating an output signal of an optical transmitter according to an exemplary embodiment of the present disclosure. The figure illustrates that the light outputted from the reflective modulator (30) is multiplexed by the wavelength division multiplexing/demultiplexing unit (20) and then is outputted to the transmission fiber (50) through the circulator (40). As illustrated in the figure, the RZ optical signal is being outputted.

FIGs. 5 and 6 are exemplary views for describing performance of a light source unit according to an exemplary embodiment of the present disclosure. In FIG. 5, A illustrates a relative intensity noise (RIN) in respect to ASE injection power when the BLS (100) illustrated in FIG.1 is linear-polarized. B illustrates a RIN in respect to ASE injection power when the BLS (100) illustrated in FIG.1 is unpolarized. C illustrates a RIN of the BLS (10) in respect to ASE injection power according to an exemplary embodiment of the present disclosure.

As illustrated in the figure, 4dB of noise is reduced in respect to the conventional BLS (A), and 1dB of noise is reduced in respect to the unpolarized BLS (B). That is, according to an exemplary embodiment of the present disclosure, 3dB of injection light power may be lowered when intending to acquire a RIN identical to that of the unpolarized BLS (B).

Meanwhile, in FIG. 6, D illustrates a RIN in respect to frequency of a polarized BLS. E illustrates a RIN in respect to frequency of an unpolarized BLS. G illustrates a RIN in respect to the BLS (10) according to an exemplary embodiment of the present disclosure. F illustrates a RIN of input signal in the transmitter. All of the four cases in the above are measured at -12dBm injection light power.

When an ASE light source is injected in a reflective modulator, a gain saturation phenomenon where the gain decreases as intensity of the injected light grows higher occurs. Thus, as illustrated in the figure, in respect to a unpolarized BLS (E), the noise at low frequency band is reduced by around 10dB compared to a polarized BLS (D), however, the noise at high frequency band may not be effectively suppressed. According to the light source unit (10) as in an exemplary embodiment of the present disclosure, the noise characteristics across a broad frequency band may be enhanced at around 5.35GHz band by the interferometric noise suppressing unit (13).

FIG. 7 is a block diagram illustrating an optical transmitter according to another exemplary embodiment of the present disclosure. The difference to the exemplary embodiment as in FIG. 3 is that an unpolarized BLS (16) is used as a light source and a polarization separating unit (17) is further arranged at the first stage. Thus, descriptions for the other structural elements will not be omitted.

According to another exemplary embodiment of the present disclosure, an EDFA may be used as an unpolarized BLS (16). As the EDFA has two perpendicular polarized lights, only one polarized light may be separated by the polarization separating unit (17) and be inputted to the second stage. Meanwhile, the other polarized light separated by the polarization separating unit (17) may be used for another transmitter.

FIGs. 8 and 9 are exemplary embodiment block diagrams illustrating a WDM system according to the present disclosure. FIG. 8 is for describing a 10Gb/s downstream signal transmission, and FIG. 9 is for describing an upstream signal transmission.

As illustrated in the figure, in a WDM system where the optical transmitter (1) according to an exemplary embodiment of the present disclosure is used, the demultiplexing unit (2) demultiplexes the light transmitted from the optical transmitter (1) as a head-end, and then transmits the demultiplexed light to a plurality of tail-end-equipment (TEE)s. According to an exemplary embodiment of the present disclosure, as the wavelength division multiplexing/demultiplexing unit (20) multiplexes the light into 40 wavelengths and then transmits to 40 TEEs, the TEEs are described as 40 units. In addition, although for describing in brief, only one unit of head-end equipment (HEE) is illustrated in the figure, it is apparent that 40 units of HEEs may transmit to the TEEs, in technical fields to which the present disclosure is relating.

Although the polarized BLS (11) as in FIG. 3 is illustrated as being used in FIG. 8, it is apparent that the unpolarized BLS (16) as in FIG. 7 also may be used.

A pulse is injected in the CW ASE outputted from the BLS (11) by the pulse injection unit (12). The distributing unit (13A) distributes the pulse-injected light by identical light intensity. Then the light is combined for polarized lights to be perpendicular after a delay of one pulse cycle, and is spectrum-splitted by the pre-filter (14), and is amplified by the light amplifying unit (15) to be outputted.

This outputted light is inputted to the wavelength division multiplexing/demultiplexing unit (20) by the circulator (40), and then may be demultiplexed by the wavelength division multiplexing/demultiplexing unit (20) to be inputted to the reflective modulator (30).

The reflective modulator (30) may be a SOA-REAM where an semiconductor optical amplifier (SOA) and a reflective electro-absorption modulator (REAM) are accumulated. The SOA suppresses noise of the inputted light, and then amplifies and reflects the light. The REAM modulates the relevant light with a NRZ signal of 10.7Ghz. Because the light inputted to the SOA-REAM is a pulsed-BLS, the NRZ signal may be outputted in RZ signal format.

The RZ optical signal of which data signal is modulated through the reflective modulator (30) is multiplexed through the wavelength division multiplexing/demultiplexing unit (20), and then disseminated to the single mode fiber (SMF) through the dispersion compensating fiber (DCF) and the optical amplifier. The RZ optical signal demultiplexed by the demultiplexing unit (2) may be transmitted to the optical receiver (70) of the TEE.

Meanwhile, FIG. 9 is for describing an upstream signal transmission. The pulsed-BLS outputted from the light source unit (10) is demultiplexed in the multiplexing/demultiplexing unit (2) by the circulator (40) through the DCF and the SMF, and then modulated in RZ format by the reflective modulator (30) provided to the TEE (3). The modulated plused-BLS is multiplexed in the multiplexing/demultiplexing unit (2), and then may be multiplexed by the wavelength division multiplexing/demultiplexing unit (20) through the circulator (40) in the head-end (1) to be received by the receiver (60).

FIGs. 10a and 10b are exemplary views illustrating bit-error rate (BER)s for describing 10Gb/s transmission characteristics by injection lights sources. FIG. 10a illustrates BER curves by injection powers when an unpolarized CW-BLS is used as an injection light source and the signal transmission format is NRZ. FIG. 10b illustrates BER curves by injection powers when a pulsed-BLS according to an exemplary embodiment of the present disclosure is used as an injection light source and the signal transmission format is RZ.

As illustrated in FIG. 10a, the conventional optical transmitter has a power penalty of 9dB in order to satisfy the first generation FEC threshold (FECth) of 1.8×10-4 at -18dBm injection power(the minimum injection power defined by ITU-T international standard G.698.3). However, the optical transmitter according to an exemplary embodiment of the present disclosure may satisfy the FECth at -18dBm injection power with a power penalty of 2dB. That is to say, according to an exemplary embodiment of the present disclosure, the power penalty may be enhanced by 7dB compared to the conventional CW-BLS/NRZ.

FIG. 11 is an exemplary view for comparing performances of pulsed-BLSs by injection light sources. The figure illustrates pulsed-BLSs where the interferometric noise suppressing unit (MZI) (13) according to an exemplary embodiment of the present disclosure is used, compared to polarized pulsed-BLSs and unpolarized pulsed-BLSs where the interferometric noise suppressing unit (MZI) (13) according to an exemplary embodiment of the present disclosure is not used.

As illustrated in the figure, compared to the unpolarized pulsed-BLS, the pulsed-BLSs where the interferometric noise suppressing unit (MZI) (13) according to an exemplary embodiment of the present disclosure is used may lower the injection power by around 3dB in order to acquire the identical received power. In addition, whereas the polarized pulsed-BLS is hardly able to satisfy the FECth even in respect to 12dBm injection power, the pulsed-BLSs where the interferometric noise suppressing unit (MZI) (13) according to an exemplary embodiment of the present disclosure has the advantage of providing the identical performance irrespective of polarized BLS or unpolarized BLS.

The abovementioned exemplary embodiments are intended to be illustrative, and not to limit the scope of the claims. Many alternatives, modifications, variations, and equivalents will be apparent to those skilled in the art. The features, structures, methods, and other characteristics of the exemplary embodiments described herein may be combined in various ways to obtain additional and/or alternative exemplary embodiments. Therefore, the technical scope of the rights for the present disclosure shall be decided by the claims.

Claims

1. A light source unit, comprising:

a broadband light source (BLS);

an injection unit configured to inject a pulse into an output of the BLS to output a pulsed light;

an interferometric noise suppressing unit configured to:

distribute the pulsed light into first and second outputs,

delay the first output, and

combine the second output and delayed first output; and

a light amplifying unit configured to amplify output from the interferometric noise suppressing unit.

The light source unit of claim 1, further comprising:

an polarization beam splitter configured to polarize the pulsed light and to distribute the pulsed light.

The light source unit of claim 1, wherein the BLS outputs an unpolarized light, wherein the light source further includes a light separating unit configured to separate any one of the polarized lights from the unpolarized light.

The light source unit of claim 1, wherein the injection unit includes:

a pulse providing unit configured to provide a clock signal having a predetermined frequency, and

a modulating unit configured to modulate an output from the BLS using the clock signal.

The light source unit of claim 4, wherein the injection unit further includes:

a first control unit configured to control polarization of inputted light so that polarization of the inputted light and polarization of the modulation unit are in a same direction, by being arranged in a front end of the modulating unit.

The light source of claim 1, wherein the interferometric noise suppressing unit includes a distributing unit configured to distribute inputted lights to a first path and a second path, a light delaying unit configured to delay a light of the second path by one pulse cycle over a light of the first path, and a combining unit configured to combine the light of the first path with the light of the second path delayed by the light delaying unit.

The light source of claim 6, wherein the interferometric noise suppressing unit further includes second and third control units configured to respectively control polarizations of the lights of the first path and the second path so as to be perpendicular to each other.

The light source of claim 1, further comprising a filtering unit configured to spectrum-split a light inputted in a predetermined band, by being arranged at a front end of the amplifying unit.

The light source unit of claim 1, wherein the BLS includes superluminescent diode (SLD) or Fabry-Perot laser diode (F-P LD).

An optical transmitter, comprising:

the light source unit of claim1;

a demultiplexing unit configured to demultiplex a pulse-seeded light inputted from the light source unit;

a plurality of reflective modulators configured to modulate a pulse-seeded light received from the demultiplexing unit into an electrical NRZ signal; and

a multiplexing unit configured to multiplex a plurality of signals inputted from the plurality of reflective modulators.

The optical transmitter of claim 10, wherein the reflective modulator outputs an RZ optical signal.

The optical transmitter of claim 10, further comprising:

a synchronizing unit configured to synchronize a light inputted to the reflective modulator with an electrical NRZ signal.

The optical transmitter of claim 10, further comprising:

a suppressor configured to suppress the intensity noise by being arranged at a front end of the plurality of reflective modulators.