US20150372763A1 - Laser transceiver with improved bit error rate - Google Patents
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- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/50—Transmitters
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- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/40—Transceivers
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- H—ELECTRICITY
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- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/50—Transmitters
- H04B10/501—Structural aspects
- H04B10/503—Laser transmitters
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- H04B—TRANSMISSION
- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/50—Transmitters
- H04B10/501—Structural aspects
- H04B10/503—Laser transmitters
- H04B10/504—Laser transmitters using direct modulation
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Abstract
Description
- The present invention relates to laser transceivers, and more particularly, to an injection locked laser transceiver with crossing point adjustment circuitry for improved bit error rate for use in a wavelength division multiplexed passive optical network.
- Optical communications networks, at one time, were generally “point to point” type networks including a transmitter and a receiver connected by an optical fiber. Such networks are relatively easy to construct but deploy many fibers to connect multiple users. As the number of subscribers connected to the network increases and the fiber count increases rapidly, deploying and managing many fibers becomes complex and expensive.
- A passive optical network (PON) addresses this problem by using a single “trunk” fiber from a transmitting end of the network, such as an optical line terminal (OLT), to a remote branching point, which may be up to 20 km or more. One challenge in developing such a PON is utilizing the capacity in the trunk fiber efficiently in order to transmit the maximum possible amount of information on the trunk fiber. Fiber optic communications networks may increase the amount of information carried on a single optical fiber by multiplexing different optical signals on different wavelengths using wavelength division multiplexing (WDM). In a WDM-PON, for example, the single trunk fiber carries optical signals at multiple channel wavelengths to and from the optical branching point and the branching point provides a simple routing function by directing signals of different wavelengths to and from individual subscribers. At each subscriber location, an optical networking terminal (ONT) or optical networking unit (ONU) is assigned one or more of the channel wavelengths for sending and/or receiving optical signals.
- A challenge in a WDM-PON, however, is designing a network that will allow the same transmitter to be used in an ONT or ONU at any subscriber location. For ease of deployment and maintenance in a WDM-PON, it is desirable to have a “colorless” ONT/ONU whose wavelength can be changed or tuned such that a single device could be used in any ONT/ONU on the PON. With a “colorless” ONT/ONU, an operator only needs to have a single, universal transmitter or transceiver device that can be employed at any subscriber location.
- One or more tunable lasers may be used to select different wavelengths for optical signals in a WDM system or network such as a WDM-PON. Various different types of tunable lasers have been developed over the years, but most were developed for high-capacity backbone connections to achieve high performance and at a relatively high cost. Less expensive tunable lasers have been developed, such as, for example the injection locked (IL) laser which is seeded by a filtered broadband light source (BLS). The IL laser is effectively tuned to the wavelength associated with the pass band of the BLS filter. The IL laser, however, is typically noisier than other, more expensive, tunable lasers and lacks the linearity properties of those more expensive lasers. This can cause distortion of the pulse width of the modulating signal which results in an increased communication bit error rate (BER).
- These and other features and advantages will be better understood by reading the following detailed description, taken together with the drawings wherein:
-
FIG. 1 is a top level schematic diagram of a wavelength division multiplexed (WDM) optical communication system including at least one transceiver, consistent with embodiments of the present disclosure. -
FIG. 2 is a schematic diagram of a wavelength division multiplexed (WDM) passive optical network (PON) including at least one transceiver, consistent with embodiments of the present disclosure. -
FIG. 3 is a schematic diagram of an optical transceiver with improved bit error rate, consistent with embodiments of the present disclosure. -
FIG. 4 is a signal diagram illustrating an eye pattern, consistent with an embodiment of the present disclosure. -
FIG. 5 is a schematic diagram of a crossing point control circuit of an optical transceiver, consistent with another embodiment of the present disclosure. - A laser transceiver with improved bit error rate, consistent with embodiments described herein, generally includes an injection locked (IL) laser transmitter module with driver circuitry configured to adjust the crossing point of the modulating RF signal to reduce distortion during transmission. The transceiver may also include a receiver module with a low-pass filter to reduce high frequency received noise, and a decision threshold circuit configured to lower the received signal decision threshold to a level where noise is reduced. The adjustments of the crossing point for transmission and the decision threshold for reception may be adaptively set and/or updated by a microcontroller or other processor based on operating characteristics of the system, such as, for example, the type of IL laser being used.
- The laser transceiver may be used in a wavelength division multiplexed (WDM) passive optical network (PON). The transceiver may be incorporated, for example, in an optical networking terminal (ONT), optical line terminal (OLT) or optical networking unit (ONU) of the WDM PON. The reduction of noise and distortion may lower communication bit error rates and improve communication over the optical network.
- As used herein, “channel wavelengths” refer to the wavelengths associated with optical channels and may include a specified wavelength band around a center wavelength. In one example, the channel wavelengths may be defined by an International Telecommunication (ITU) standard such as the ITU-T dense wavelength division multiplexing (DWDM) grid. As used herein, “tuning to a channel wavelength” refers to adjusting a laser output such that the emitted laser light includes the channel wavelength. The term “coupled” as used herein refers to any connection, coupling, link or the like and “optically coupled” refers to coupling such that light from one element is imparted to another element. Such “coupled” devices are not necessarily directly connected to one another and may be separated by intermediate components or devices that may manipulate or modify such signals.
- Referring to
FIG. 1 , a WDMoptical communication system 100 including one ormore transceivers 102 with reduced bit error rate, consistent with embodiments of the present disclosure, is shown and described. TheWDM system 100 includes one ormore terminals path 114 for transmitting and receiving optical signals at different channel wavelengths over the trunkoptical path 114.Terminal 110 may be an optical line terminal (OLT) whileterminals terminals WDM system 100 include one ormore transceivers 102 which further include transmitters 120 (e.g., TX1 to TXn) and receivers 122 (e.g., RX1 to RXn) associated with different channels (e.g., Ch. 1 to Ch. n) for transmitting and receiving optical signals at the different channel wavelengths between the one ormore terminals - Each
terminal more transmitters 120 andreceivers 122, and thetransmitters 120 andreceivers 122 may be separate or integrated as a transceiver within a terminal. Optical multiplexers/demultiplexers WDM system 100 combine and separate the optical signals at the different channel wavelengths. Aggregate WDM optical signals including the combined channel wavelengths are carried on the trunkoptical path 114. One or more of thetransmitters 120 may be a tunable transmitter capable of being tuned to the appropriate channel wavelength through injection locking based on seeding from a broadband light source, as will be described in greater detail below. Thus, thetransmitters 120 may be constructed as universal transmitters capable of being used in different locations in theWDM system 100 and tuned to the appropriate channel wavelength depending upon the location in theWDM system 100. - Referring to
FIG. 2 , an embodiment of the WDM optical communication system ofFIG. 1 is shown in greater detail. One or more transceivers, consistent with embodiments of the present disclosure, may be used to transmit and receive optical signals in a WDM-PON 200. The WDM-PON 200 provides a point-to-multipoint optical network architecture using a WDM system. According to one embodiment of the WDM-PON 200, at least one optical line terminal (OLT) 110 may be coupled to a plurality of optical networking terminals (ONTs) or optical networking units (ONUs) 111, 112, . . . , etc. via optical fibers, waveguides, and/orpaths 114. The OLT 110 and the ONUs 111, 112 include one or moreoptical transceivers 102 configured to provide reduced bit error rates, as described in greater detail below. - The OLT 110 may be located at a central office of the WDM-
PON 200, and the ONUs 111, 112 may be located in homes, businesses or other types of subscriber location or premises. Theoptical demultiplexer 118, or branching point, may be configured to couple a trunkoptical path 114 to separate optical paths to theONUs - One application of the WDM-
PON 200 is to provide fiber-to-the-home (FTTH) or fiber-to-the-premises (FTTP) capable of delivering voice, data, and/or video services across a common platform. In this application, the central office may be coupled to one or more sources or networks providing the voice, data and/or video. - In the WDM-
PON 200, different ONUs 111, 112 may be assigned different channel wavelengths for transmitting and receiving optical signals. In one embodiment, the WDM-PON 200 may use different wavelength bands for transmission of downstream and upstream optical signals relative to theOLT 110 to avoid interference between the received signal and back reflected transmission signal on the same fiber. For example, the L-band (e.g., about 1565 to 1625 nm) may be used for downstream transmissions from theOLT 110 and the C-band (e.g., about 1530 to 1565 nm) may be used for upstream transmissions to theOLT 110. The upstream and/or downstream channel wavelengths may generally correspond to the ITU grid. In one example, the upstream wavelengths may be aligned with the 100 GHz ITU grid and the downstream wavelengths may be slightly offset from the 100 GHz ITU grid. - The
ONUs ONUs ONUs - One embodiment of the
ONUs transceiver 102 comprising an (IL) laser for transmitting an optical signal at the assigned channel wavelength (λC1) and a photodetector, such as a photodiode, for receiving an optical signal at the assigned downstream channel wavelength (λL1). - The
OLT 110 may be configured to generate multiple optical signals at different channel wavelengths (e.g., λL1, λL2, . . . λLn) and to combine the optical signals into the downstream WDM optical signal carried on the trunk optical fiber orpath 114. TheOLT 110 may also be configured to separate optical signals at different channel wavelengths (e.g., λC1, λC2, . . . λCn) from an upstream WDM optical signal carried on thetrunk path 114 and to receive the separated optical signals. - Transceivers or transmitters located within the
OLT 110 may be configured to transmit an optical signal on at least one channel wavelength in the L-band (e.g., λL1, λL2, . . . λLn) based on seeding of the laser as will be explained in greater detail below. Other wavelengths and wavelength bands are also within the scope of the system and method described herein. - The IL lasers of
transceivers 102 may be modulated by RF data signals to generate the respective optical signals. The lasers may be modulated using various modulation techniques including external modulation and direct modulation. - In one embodiment, one or more broadband light sources (BLSs), for example a C-
band BLS 232 and an L-band BLS 234, may be configured to generate broadband light over a desired wavelength range such as the C-band or the L-band, respectively. The broadband light generated bymodule optical coupler 236, into thetrunk path 114 such that L-band seeding is provided to theOLT 110 and C-band seeding is provided toONUs L filter modules 230 may be provided in the path to eachtransceiver 102 and configured to separate incoming C-band (or L-band) wavelength light from outgoing L-band (or C-band) wavelength light respectively. Thus, for example, thereceivers 122 of eachtransceiver 102 ofOLT 110 will receive the appropriate C-band signal wavelength assigned to that receiver. Likewise, the IL lasers of eachtransmitter 120 of eachtransceiver 102 of the OLT will receive the appropriate L-band wavelength seeding signal so that the IL laser may transmit at the assigned wavelength within the L-band. - Similarly, for example, the
receivers 122 of eachtransceiver 102 ofONUs transmitter 120 of eachtransceiver 102 of the ONUs will receive the appropriate C-band wavelength seeding signal so that the IL laser may transmit at the assigned wavelength within the C-band. - Referring to
FIG. 3 , a transceiver with improved bit error rate is described in greater detail. In some embodiments, thetransceiver 102 includes a transmitter component 330 (e.g.,TX 120 ofFIG. 1 ) and a receiver component 340 (e.g.,RX 122 ofFIG. 1 ), either or both of which may be under the control of a processor or micro-controller unit (MCU) 312, as will be explained below. - The
transmitter component 330 may include anIL laser diode 306 configured to generate laser light in a desired wavelength range for transmission over an optical network, for example theWDM PON 200. The IL laser is considered to be a “colorless” laser because it does not have a predefined lasing wavelength, but rather it lases at the wavelength of an injected seeding light and may lock onto the injected seeding light over a relatively wide range of wavelengths. Thelaser diode 306 is seeded by a broadband light source (BLS) 308 that is filtered by aWDM PON filter 310 which is configured as a narrow band-pass optical filter. TheBLS 308 may emit light that covers a wide range of wavelengths. Thefilter 310 is configured to filter the light provided by theBLS 308 down to a wavelength range that corresponds to the desired wavelength range for thelaser 306 and thus seeds the laser for transmission at that wavelength. In some embodiments, thefilter 310 may be a thin-film filter or an array waveguide grating (AWG). TheBLS 308 may correspond, for example, to the C-band BLS 232 and/or the L-band BLS 234 ofFIG. 2 . TheWDM PON filter 310 may be incorporated, for example, in the optical multiplexer/demultiplexer (e.g., AWG)modules FIG. 2 . - Laser
diode driver circuit 304 is electrically coupled tolaser 306 and may be configured to drive the laser by applying a driving current to induce lasing. Thelaser driver circuit 304 may modulate thelaser 306 with an electrical signal that represents the signal intended for transmission,Tx Data 332, which will typically be provided as a radio frequency (RF) signal. Thedriver 304 thus causes thelaser 306 to generate a modulated optical signal for transmission at the desired channel wavelength. The crossingpoint control circuit 302 may be configured to adjust the waveform shape of the Tx Data signal 332 to improve the transmission characteristics of the signal, as explained below. - The TX data signal 332 may be a binary signal (e.g., on-off keying modulated signal) having an amplitude or voltage that transitions between a first value associated with a logical ‘0’ signal level and a second value associated with a logical ‘1’ signal level, as illustrated in
FIG. 4( a), which is commonly referred to as an “eye” diagram. The signal shown inFIG. 4( a) is relatively clean and symmetric (e.g., the crossing point being approximately halfway between the two signal levels). In such a case it may be straightforward to distinguish a ‘1’ from a ‘0’ after transmission and reception of the modulated optical signal. Unfortunately, due to the nature of theBLS 308, which is typically an amplified spontaneous emission (ASE) light source, the light from anIL laser 306 is generally noisier than the light produced by a more expensive distributed feedback (DFB) or Fabry-Perot (FP) laser. Additionally, the fabrication techniques for an IL laser may result in a laser chip design having a longer dimensional length, which may adversely affect the linearity of the IL laser. This non-linearity may shift the crossing point of the eye diagram down towards the ‘0’ level causing pulse width distortion (e.g., the pulse width of the ‘0’ signal is different from the pulse width of the ‘1’ signal) resulting in communication errors (e.g., higher bit error rates).FIG. 4( b) illustrates an example of such a noisier and distorted signal. - The crossing
point control circuit 302 may be configured to adjust the waveform shape of the Tx Data signal 332, used to modulate/drive the laser, by pre-distorting the signal to shift the crossing point to a higher value or level. This pre-distortion may, at least partially, compensate for the subsequent signal distortion introduced by the non-linear characteristics of the IL laser. The resulting transmitted optical signal may therefore have a crossing point closer to the desired halfway point between the level ‘1’ and level ‘0’ signals. The amount of pre-distortion may be controlled by theMCU 312 and may depend on the characteristics of the particular IL laser being used, for example measured or otherwise known distortion, and/or any other relevant factors. - The
receiver component 340 may include aphotodetector 320 configured to receive an optical signal from an optical network, for example theWDM PON 200. The received signal may also be a binary signal (e.g., on-off keying modulated signal). In some embodiments, the photodetector may include a trans-impedance amplifier to provide an initial amplification of the received signal before subsequent processing operations are performed. The photodetector converts the received optical signal into an electrical signal, which may, for example, be in the RF frequency range. A low-pass filter 318 may process the output of thephotodetector 320 to limit the bandwidth of the received signal and remove the higher frequency noise that may have been introduced by the IL laser and/or the transmission through the optical network. The low pass filter may have a cut-off frequency, above which noise is filtered. In some embodiments, the cut-off frequency may be fixed or adjustable. - A
decision threshold circuit 316 may be configured to set a threshold for determining whether the received signal represents a logical ‘0’ signal level or a logical ‘1’ signal level.FIG. 4( b) illustrates anexample decision threshold 402. In the absence of noise, distortion or other undesirable interference, the decision threshold might be set to approximately 50 percent of the full scale signal amplitude or approximately halfway between the expected signal amplitude associated with a level ‘1’ and a level ‘0.’ However, in practice, a lower decision threshold may improve receiver performance since more noise is typically associated with the ‘1’ level due to the operating characteristics of the IL laser. In some preferred embodiments, a decision threshold in the range of approximately 20 to 30 percent of the full scale signal amplitude (e.g., the expected signal amplitude associated with a level ‘1’). - In some embodiments, the decision threshold may be adaptively set in response to changing characteristics or conditions of the transceiver system and/or the optical network. The threshold may be set, for example, by the
MCU 312. - In some embodiments, the decision threshold adjustment may be performed as part of the post-amplifier circuit or
module 314 which is configured to provide the received datasignal RX Data 342 to the ONU or OLT as, for example, an RF signal in a desired voltage range. - The crossing
point control circuit 302 anddecision threshold circuit 316 may be under the control of a processor orMCU 312 which may receive data/commands, for example over a digital bus 350, from an external entity that is employing thetransceiver 102. In some embodiments, the digital bus may conform to the inter-integrated circuit (I2C) standard or the small form factor (SFF) multi-source agreement (MSA) standard. For example, the MCU may be configured to receive a request or instruction to adjust the crossing point of the modulating transmit signal or adjust the decision threshold of the received signal. In response to that request, the MCU may generate the control signals necessary to achieve these conditions and provide these control signals to the crossingpoint control circuit 302 and/or thedecision threshold circuit 316. The MCU may operate based on software execution/programming, firmware, hardware or any combination thereof. - In some embodiments, the
transceiver circuit 102 may conform to the dimensions of the Small Form Factor (SFF) or a Small Form Factor Pluggable (SFP) transceiver size specification. These dimensions are set forth, for example, in the “Small Form Factor Transceiver Multisource Agreement,” dated Jan. 6, 1998, and the “Small Form Factor Pluggable Transceiver Multisource Agreement,” dated Sep. 14, 2000. It will be appreciated that the bit error rate reduction techniques described herein, which enable the use of the relatively less complex IL laser, allows for a decrease in size (and cost) of the transceiver. This may contribute, at least in part, to the ability to conform to the SFF/SFP specification. - Referring to
FIG. 5 , a schematic diagram of one example embodiment of the crossingpoint control circuit 302 is shown in greater detail.Differential driver circuit 502 may be configured to convert the Tx Data signal 332 into a differential version of that signal (e.g, Tx+ and Tx−) each of which is coupled to abias circuit MCU 312 provides bias balance signals 508 to each of thebias circuits differential signal 510 may be adjusted to a higher value. - Accordingly, an optical transceiver, with signal crossing point control and decision threshold circuitry, consistent with embodiments described herein, may provide communications with reduced bit error rate over a WDM PON. The optical transceiver may use a relatively inexpensive IL laser and may conform to a relatively small form factor.
- Consistent with one embodiment, an optical transceiver generally includes an injection locked (IL) laser configured to generate a transmit (Tx) optical signal for transmission over an optical network and a laser driver circuit configured to modulate the IL laser based on a Tx data signal. The Tx data signal may be provided to the optical transceiver for transmission over the optical network. The Tx data signal may include a crossing point level associated with a transition between a first signal level and a second signal level. The optical transceiver may also include a crossing point control circuit configured to apply distortion to the Tx data signal, the distortion to increase the crossing point level.
- Consistent with another embodiment, an optical networking unit includes an injection locked (IL) laser configured to generate a transmit (Tx) optical signal for transmission over an optical network at a transmission channel wavelength, wherein the transmission channel wavelength is in one of the L-band or the C-band. The ONU also includes a laser driver circuit configured to modulate the IL laser based on a Tx data signal, the Tx data signal provided to the optical transceiver for transmission over the optical network. The Tx data signal includes a crossing point level associated with a transition between a first signal level and a second signal level. The ONU further includes a crossing point control circuit configured to apply distortion to the Tx data signal, the distortion to increase the crossing point level. The ONU further includes a photodetector configured to convert a received (Rx) optical signal from the optical network to an electrical Rx data signal, the Rx optical signal received at an Rx channel wavelength in one of the L-band or the C-band.
- Consistent with a further embodiment, a wavelength division multiplexed (WDM) system includes a plurality of terminals associated with different respective channel wavelengths and configured to transmit optical signals on the different respective channel wavelengths. At least one of the plurality of terminals includes at least an optical transceiver. The optical transceiver includes an injection locked (IL) laser configured to generate a transmit (Tx) optical signal for transmission over an optical network. The optical transceiver also includes a laser driver circuit configured to modulate the IL laser based on a Tx data signal, the Tx data signal provided to the optical transceiver for transmission over the optical network. The Tx data signal includes a crossing point level associated with a transition between a first signal level and a second signal level. The optical transceiver further includes a crossing point control circuit configured to apply distortion to the Tx data signal, the distortion to increase the crossing point level.
- Consistent with yet another embodiment, a method includes providing an injection locked (IL) laser configured to generate a transmit (Tx) optical signal for transmission over an optical network. The method also includes modulating the IL laser based on a Tx data signal, the Tx data signal provided to the optical transceiver for transmission over the optical network. The Tx data signal includes a crossing point level associated with a transition between a first signal level and a second signal level. The method further includes applying distortion to the Tx data signal to increase the crossing point level. The method further includes converting a received (Rx) optical signal from the optical network to an electrical Rx data signal; and adjusting a threshold for determining whether the Rx data signal corresponds to the first signal level or the second signal level.
- While the principles of the invention have been described herein, it is to be understood by those skilled in the art that this description is made only by way of example and not as a limitation as to the scope of the invention. Other embodiments are contemplated within the scope of the present invention in addition to the exemplary embodiments shown and described herein. Modifications and substitutions by one of ordinary skill in the art are considered to be within the scope of the present invention, which is not to be limited except by the following claims.
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EP15810856.3A EP3161982A4 (en) | 2014-06-24 | 2015-06-24 | Laser transceiver with improved bit error rate |
PCT/US2015/037316 WO2015200420A1 (en) | 2014-06-24 | 2015-06-24 | Laser transceiver with improved bit error rate |
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Also Published As
Publication number | Publication date |
---|---|
EP3161982A4 (en) | 2018-03-07 |
CN107078830A (en) | 2017-08-18 |
US9236949B1 (en) | 2016-01-12 |
WO2015200420A1 (en) | 2015-12-30 |
EP3161982A1 (en) | 2017-05-03 |
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