Apparatus and method for generating optical signals in an optical communication network

Abstract

The invention discloses a device and method for generating an optical signal in an optical network. The device comprises at least: a first and a second laser and a polarization beam combiner (PBC) for generating a first or second series of optical pulses, respectively. The PBC is used to receive the first and second series of optical pulses. Each optical pulse in the first series of optical pulses is received with a first state of polarization (SOP), and each optical pulse in the second series of optical pulses is received with a second SOP. The first SOP and the second SOP are orthogonal to each other, and the PBC is also used to combine the first and second series of optical pulses and output a non-polarized optical signal. The non-polarized optical signal can help reduce or mitigate the nonlinear optical damage caused by the polarization of the optical signal.

Description

[0001] Related Applications

[0002] This is the first application of the present invention. Technical Field

[0003] The present application relates to optical communication networks, and in particular to apparatus and methods for mitigating nonlinear impairments in optical systems. Background Art

[0004] In optical communication networks, an optical time-domain reflectometer (OTDR) is an optical test instrument used to detect the optical return loss of an optical fiber being tested or monitored (also called a monitoring fiber). The OTDR sends an optical signal consisting of a series of optical pulses into the monitoring fiber and detects the reflected signal that is scattered (e.g., Rayleigh backscattering) and / or reflected back along the monitoring fiber. The detected reflected signal is used to characterize the monitoring fiber. In some applications, the OTDR is the optical equivalent of an electronic time domain reflectometer, which measures reflections caused by impedance changes in the cable under test. The reflected signal strength is measured as a function of time and plotted as a function of the length of the tested monitoring fiber or cable.

[0005] Therefore, an OTDR with higher accuracy, measurement range, and instrument resolution is highly desirable in testing and troubleshooting solutions in optical communication networks. For example, the dynamic range of an OTDR is an important parameter for evaluating the performance of an OTDR. The dynamic range is defined as the difference between the initial intensity of the reflected signal (at the near end of the fiber under test) and the detection noise floor (the sum of all noise sources and unwanted signals). The maximum fiber length that can be monitored depends on the dynamic range of the OTDR. In order to increase the dynamic range of the OTDR, the OTDR can generate a series of high-power optical pulses. However, the use of a series of high-power optical pulses may cause different types of impairments to the optical signal carrying the payload data sent on the monitored optical fiber. These impairments caused may be nonlinear optical effects, including cross-phase modulation (XPM) and cross-polarization modulation (XPolM), which largely depend on the state of polarization (SOP) of the series of optical pulses. Such impairments can degrade the performance of other optical signals carrying payload data along the optical fiber.

[0006] Some conventional methods are used to reduce nonlinear optical effects on other optical signals sent on the monitoring optical fiber. For example, one known technique is to make the power of each optical pulse generated from the OTDR have a lower power, which may result in a smaller dynamic range of the OTDR. Another conventional method uses the OTDR in an offline manner (e.g., by disconnecting the components of the optical communication network to be tested), which may interrupt the service being provided by the optical communication network.

[0007] Therefore, it is desirable to provide an optical device for detecting the performance of an optical communication network (e.g., including an optical fiber or optical fiber link that requires testing, measurement, or certification) while reducing nonlinear optical impairments to other optical signals carrying payload data in the optical communication network and without disrupting services provided by the optical communication network. Summary of the invention

[0008] The present invention provides apparatus and methods for generating non-polarized light signals. In certain embodiments, non-polarized light signals are generated by combining first and second series of light pulses. Each light pulse in the first series of light pulses has a first SOP, and the first SOP is orthogonal to the second SOP of each light pulse in the second series of light pulses. Because the light signal generated comprises dual polarization (for example, orthogonal first SOP and second SOP), the light signal generated thus behaves as being similar to non-polarized signal.

[0009] According to some exemplary aspects, the present invention describes a device in an optical network. The device includes at least a first and a second laser and a polarization beam combiner (PBC). The first laser is used to generate a first series of optical pulses, and the second laser is used to generate a second series of optical pulses. The PBC is used to receive the first and second series of optical pulses. Each optical pulse in the first series of optical pulses is received with a first state of polarization (SOP), and each optical pulse in the second series of optical pulses is received with a second SOP. The first SOP and the second SOP are orthogonal to each other, and the PBC is also used to combine the first and second series of optical pulses and output a non-polarized light signal.

[0010] In any of the preceding aspects / embodiments, a first frequency of the first series of light pulses generated from the first laser has a frequency offset relative to a second frequency of the second series of light pulses generated from the second laser.

[0011] In any of the foregoing aspects / embodiments, each light pulse in at least one of the first and second series of light pulses has a corresponding rising edge and a corresponding falling edge. The first laser is used to adjust at least one of a rising slope time on the corresponding rising edge or a falling slope time on the corresponding falling edge to be greater than zero for the light pulses in the first series of light pulses. The second laser is used to adjust at least one of a rising slope time on the corresponding rising edge or a falling slope time on the corresponding falling edge to be greater than zero for the light pulses in the second series of light pulses.

[0012] In any one of the aforementioned aspects / embodiments, the device also includes at least a first and a second optical fiber. The first optical fiber couples the first laser to the PBC, and the second optical fiber couples the second laser to the PBC. The first optical fiber is arranged so that the first optical fiber has a first orientation physically rotated 90 ° relative to the second orientation of the second optical fiber, so that the first SOP of each of the first series of light pulses received at the PBC is orthogonal to the second SOP of each of the second series of light pulses received at the PBC.

[0013] In any of the preceding aspects / embodiments, each light pulse in the first series of light pulses has at least one of a power, a pulse shape, or a timing equal to each light pulse in the second series of light pulses.

[0014] In any of the aforementioned aspects / embodiments, the apparatus further comprises an optical circulator for sending the unpolarized light signal into a monitoring optical fiber.

[0015] In any of the aforementioned aspects / embodiments, the device further comprises a photodetector (PD) for receiving a reflected signal scattered back along the monitoring optical fiber and measuring a power and a time delay of the reflected signal.

[0016] In any of the preceding aspects / embodiments, the apparatus comprises an optical time domain reflectometer (OTDR).

[0017] In any of the preceding aspects / embodiments, each of the first and second SOPs comprises a linear polarization.

[0018] According to some exemplary aspects, the present invention describes a method for generating an optical signal in an optical network. The method includes generating a first series of optical pulses and a second series of optical pulses; receiving the first and second series of optical pulses, each optical pulse in the first series of optical pulses is received with a first state of polarization (SOP), each optical pulse in the second series of optical pulses is received with a second SOP, the first SOP and the second SOP are orthogonal to each other; and combining the first and second series of optical pulses to output the unpolarized optical signal.

[0019] In any of the preceding aspects / embodiments, a first frequency from the first series of light pulses has a frequency offset relative to a second frequency from the second series of light pulses.

[0020] In any of the foregoing aspects / embodiments, each light pulse in at least one of the first and second series of light pulses has a corresponding rising edge and a corresponding falling edge. The method further comprises adjusting at least one of a rising slope time on the corresponding rising edge or a falling slope time on the corresponding falling edge to be greater than zero.

[0021] In any one of the aforementioned aspects / embodiments, the first series of light pulses is carried on a first optical fiber, and the first optical fiber is arranged so that the first optical fiber has a first orientation physically rotated 90 ° relative to a second orientation of a second optical fiber, so that the first SOP of each of the first series of light pulses is orthogonal to the second SOP of each of the second series of light pulses carried on the second optical fiber. In any one of the aforementioned aspects / embodiments, each light pulse in the first series of light pulses has at least one of a power, a pulse shape or a timing equal to each light pulse in the second series of light pulses.

[0022] In any of the preceding aspects / embodiments, the method further comprises sending the optical signal into a monitoring optical fiber.

[0023] In any of the foregoing aspects / embodiments, the method further comprises receiving a reflected signal scattered back along the monitoring optical fiber; and measuring a power and a time delay of the reflected signal.

[0024] In any of the preceding aspects / embodiments, each of the first and second SOPs comprises a linear polarization.

[0025] In any of the foregoing aspects / embodiments, the method is implemented on an optical time domain reflectometer (OTDR).Each variation or alternative is discussed in more detail below, along with some potential benefits. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Reference will now be made, by way of example, to the accompanying drawings which show exemplary embodiments of the present application, in which:

[0027] Figure 1A Demonstrate how to use traditional OTDR to characterize and monitor optical communication networks;

[0028] Figure 1B Shown by Figure 1A The wavelength of the optical signal generated by the traditional OTDR is Figure 1A The wavelength range occupied by the optical signals generated by the monitoring optical communication network;

[0029] Figure 2A An exemplary apparatus for characterizing and monitoring an optical communication network according to an exemplary embodiment is shown;

[0030] Figure 2B According to an exemplary embodiment, Figure 2A An exemplary optical signal generator of the apparatus;

[0031] Figure 2C An alternative exemplary embodiment is shown. Figure 2A An exemplary optical signal generator of the apparatus;

[0032] Figure 2D An alternative exemplary embodiment is shown. Figure 2A An exemplary optical signal generator of the apparatus;

[0033] Figure 3 Indicating that according to an exemplary embodiment, the Figure 2B a diagram of a series of pulses in an optical signal generated by an optical signal generator;

[0034] Figure 4 is a diagram showing that according to an exemplary embodiment, Figure 2A Device or Figure 2B Flowchart of an exemplary method for generating an optical signal performed by an optical signal generator.

[0035] The same reference numerals are used throughout the drawings to denote like elements and features.While aspects of the invention will be described in conjunction with the illustrated embodiments, it will be understood that they are not intended to limit the invention to these embodiments. DETAILED DESCRIPTION

[0036] The present invention provides an apparatus and method for generating an optical signal for characterizing a monitored optical communication network (reflecting or representing the physical characteristics of the monitored optical communication network). In some embodiments, the apparatus includes at least a first and a second laser and a polarization beam combiner (PBC) to generate an unpolarized optical signal. The apparatus can be used for an OTDR. As discussed in more detail below, a polarized signal is a signal in which a signal modeled as a wave constrains the wave to a specific direction. If a signal behaves in practice like a polarized signal, it can be considered polarized even if it is not fully polarized. Similarly, if a signal effectively behaves in practice like an unpolarized signal, it can be considered unpolarized.

[0037] In some examples, a non-polarized light signal is generated by combining a first series of light pulses generated from a first laser and a second series of light pulses generated from a second laser. Each of the first series of light pulses has a first state of polarization (SOP), and each of the second series of light pulses has a second SOP. The first SOP and the second SOP are orthogonal to each other. Orthogonality is not necessarily perfect. Although SOP represents a physical phenomenon, it can be modeled mathematically, for example, as a vector. In some applications, each light pulse in the first series of light pulses has at least one of the power, pulse shape or timing of each light pulse in the second series of light pulses that is approximately equal. Due to the generated light signal with dual polarization (for example, each first SOP is orthogonal to each second SOP), it is effectively shown as being similar to a non-polarized signal, so the nonlinear optical effect (for example, XPM or XPolM) of the polarization of the light signal generated can be significantly eliminated or reduced. Therefore, when the monitoring optical communication network is characterizing or troubleshooting, other parallel light signals carrying effective load data in the monitoring optical communication network can be unaffected by the test, and the business provided by the monitoring optical communication network is prevented from being interrupted.

[0038] Figure 1A An exemplary setup using a conventional OTDR is shown, which tests or diagnoses an exemplary monitored optical communication network 100 that provides various services to users. Figure 1A In the example of FIG. 1 , the monitoring optical communication network 100 includes at least one transmitter (Tx) 102, an optional amplifier 104, a wavelength-division-multiplexer (WDM) 108, an optical fiber 110, and other possible components not shown for simplicity. The transmitter 102 generates at least a first optical signal 112 carrying payload data. The optional amplifier 104 amplifies the first optical signal 112 and then generates at least one amplified signal. The WDM 108 then multiplexes the amplified signal onto the optical fiber 110.

[0039] When monitoring or diagnosing the optical communication network 100 is required, if the wavelength of the second optical signal generated by the conventional OTDR 106 is out-of-band (OOB), the conventional OTDR 106 can be used to test the link or node of the optical communication network 100 without interrupting the service being provided to the user (e.g., without disconnecting the link or node of the optical communication network 100). For example, each of the first optical signals 112 can have a typical C-band, including a wavelength range from 1529 nanometers (nm) to 1562 nm, such as Figure 1B If it is necessary to test or diagnose the optical communication network 100 without interrupting service, the wavelength of the second optical signal 114 must be different from or exceed the wavelength range (e.g., 1529 nm to 1562 nm) of the first optical signal 112. Figure 1B In the example of FIG. 1 , the wavelength of the second optical signal 114 is around 1510 nm, which falls outside the wavelength range of 1529 nm to 1562 nm occupied by the first optical signal 112 .

[0040] Although Figure 1A The optical communication network 100 shown in FIG. 1 provides an exemplary optical communication environment, but this is only illustrative and is not intended to be limiting. In other examples, the optical communication network 100 can be any other optical communication network and can include other components, such as OTDR 106, more than one amplifier 104, multiple optical fibers 110 and / or more than one WDM 108.

[0041] When the OTDR performs diagnostics or characterization of monitoring an optical communication network, various requirements should be met to ensure higher performance of the OTDR and / or reduce the impact on the transmission of the effective load-bearing optical signal 112. For example, dynamic range is an important factor in evaluating the performance of the OTDR. When the OTDR tests an optical fiber over a long distance, the power of the signal reflected from the far end of the optical fiber may be too low to be distinguished from noise. Therefore, in order to ensure that the OTDR can be used to perform diagnostics or monitoring of the optical fiber over a longer distance (i.e., a larger dynamic range), the power of the second optical signal 114 generated by the OTDR can be significantly increased. However, due to Kerr nonlinear interference between adjacent channels, the second optical signal 114 can cause a transient nonlinear phase change of the first optical signal 112 by introducing different types of nonlinear optical damage (e.g., XPM or XPolM) that are closely dependent on the polarization of the second optical signal 114. Specifically, the higher the power of the second optical signal 114, the more severe the nonlinear optical damage.

[0042] More importantly, when nonlinear phase changes have been introduced into the first optical signal 112 (e.g., a quadrature amplitude modulated signal) at the transmitter, a phase recovery algorithm and / or a polarization recovery algorithm may be applied at the receiver in the optical communication network 100 to track these nonlinear phase changes. However, there may be many residual phase errors at the rising and / or falling edges of each recovered optical signal, and the rate of nonlinear phase changes may be too fast to perform tracking. Therefore, when the payload data is recovered at the receiver, different types of errors may be introduced, which may result in a higher bit error ratio (BER).

[0043] Thus, the apparatus and methods disclosed herein are used to diagnose, characterize, and / or perform troubleshooting of a monitored optical communication network without requiring the monitored optical communication network to be taken offline (e.g., disconnect the monitored optical communication network to interrupt service). The apparatus can help reduce or eliminate nonlinear optical impairments caused by polarization of optical signals generated by the apparatus, and can also have a sufficiently large dynamic range.

[0044] Figure 2A A block diagram is shown of an exemplary apparatus 200 that may be used to diagnose, characterize, and / or perform a Figure 1A The apparatus 200 includes at least one optical signal generator 201. The optical signal generator 201 includes at least a first laser 202 and a second laser 204 and a polarization beam combiner (PBC) 210. The first laser 202 generates a first series of optical pulses, and the second laser 204 generates a second series of optical pulses. Each optical pulse in the first series of optical pulses has at least one of a power, a pulse shape, or a timing that is approximately equal to each optical pulse in the second series of optical pulses. In some examples, the power may be within about 1 dB. The first laser 202 is coupled to the PBC 210 through a first optical fiber 206, and the second optical fiber 208 couples the second laser 204 to the PBC 210. Then, the PBC combines the first and second series of optical pulses generated by the first laser 202 and the second laser 204, respectively, to generate a combined optical signal having dual polarization. Therefore, the optical signal generated from the optical signal generator 201 is a non-polarized optical signal.

[0045] Specifically, each pulse of the first series of light pulses received by PBC 210 has a first SOP, and each pulse of the second series of light pulses received by PCB 210 has a second SOP. The first SOP and the second SOP are orthogonal to each other. In some examples, each of the first and second SOPs includes linear polarization (wherein the light pulse is substantially planar along the propagation direction). Because the first SOP and the second SOP are oriented to be orthogonal to each other, the combined optical signal output by PBC 210 includes two orthogonal polarizations, and effectively behaves as being similar to a non-polarized light signal. Therefore, the nonlinear polarization-related optical damage caused by the optical signal generated by PBC 210 can be significantly eliminated or mitigated, because the optical signal generated by PBC 210 behaves as being similar to a non-polarized light signal. Therefore, when device 200 is used for diagnosing a monitoring optical communication network in operation or online, the dynamic range of device 200 can be increased, so that the longer optical fiber or link of the monitoring optical communication network is monitored, without producing serious nonlinear optical damage.

[0046] In some examples, the device 200 may be an OTDR, which may further include an optical circulator or optical coupler 214, a photodetector (PD) 216, and a processor 218. The unpolarized optical signal generated from the PBC 210 is coupled to a subsequent optical fiber through the optical circulator 214 to test, track, or perform troubleshooting on the monitoring optical communication network 100, for example, to monitor or diagnose one or more monitoring optical fibers / links 220 of the optical communication network 100. The PD 216 detects one or more reflected signals scattered (e.g., Rayleigh backscattering) or reflected back by the monitoring optical fiber 220. The processor 218 is coupled to the PD 216 to receive data representing the detected reflected optical signals from the PD 216. The processor 218 measures and processes the intensity (e.g., power) and the elapsed time (e.g., time delay) of the reflected optical signal. The processor 218 may also be coupled to a display (not shown) for outputting the measured values. For example, the processor 218 may calculate the time delay and map or convert the calculated time delay to the length of the monitoring optical fiber 220 based on the known speed of light in the optical fiber. In some examples, since the reflected signal is weak, the measurement may be repeated multiple times by using a series of light pulses in the unpolarized signal generated by the PBC 210 instead of a single light pulse.

[0047] Figure 3A diagram showing light pulses of an unpolarized signal generated by PBC 210. In some examples, a time interval 301 between two consecutive light pulses may be longer than the round trip time (RTT) required for one light pulse to travel the entire length of monitoring optical fiber 220 and return to PD 216. The time interval 301 may be in milliseconds. For example, the RTT of an optical fiber 100 kilometers (km) long is approximately 1 ms. Therefore, the time interval 301 may be longer than 1 ms.

[0048] As described above, optical pulses sent over an optical fiber for monitoring purposes may also cause undesirable nonlinear phase changes (e.g., XPM effects) in a payload-carrying optical signal simultaneously carried on the same optical fiber. When the payload-carrying optical signal is recovered at a receiver (e.g., a coherent receiver), the undesirable nonlinear phase changes may be tracked at the receiver to ensure that the recovery process is more accurate. However, when tracking the undesirable nonlinear phase changes, residual phase errors (resulting in higher BER) may occur at the rising and / or falling edges of each optical pulse of the recovered optical signal because the rate of the undesirable nonlinear phase changes is too fast to track. Therefore, it may be useful to slow down the rate of undesirable nonlinear phase changes caused by the optical pulses generated from the device 200. Doing so may help eliminate or mitigate the XPM effect (in addition to mitigating the XPM effect by using an unpolarized optical signal as described above).

[0049] Reference again Figure 2AIn some possible configurations, each of the first laser 202 and the second laser 204 may also include a first control circuit 212 (1) or a second control circuit 212 (2) (generally referred to as control circuit 212) that adjusts (e.g., slows down) a corresponding rising edge and / or falling edge of a corresponding optical pulse. For example, for each of the first and second series of optical pulses, in order to ensure that each optical pulse has a non-zero rising slope time and / or falling slope time, the first control circuit 212 (1) or the second control circuit 212 (2) adjusts at least one of the rising slope time on the rising edge or the falling slope time on the falling edge of the corresponding optical pulse to be greater than zero (e.g., greater than 0 seconds, 0 nanoseconds, etc.). Specifically, the control circuit 212 may adjust the rising slope time and / or the falling slope time to be slow enough to help achieve better tracking at the receiver. Therefore, the rate of nonlinear phase change caused by the optical pulse generated from the device 200 can be reduced, which in turn can help reduce or mitigate the residual phase error when the optical signal is recovered at the receiver. Therefore, the BER can be reduced. In this example, the first control circuit 212(1) and the second control circuit 212(2) can be used to control the internal settings (e.g., hardware, software implementation (e.g., digital control of signals), or a combination of hardware and software implementation) of the respective lasers 202, 204. In other possible configurations, the first control circuit 212(1) and the second control circuit 212(2) can be components external to the first laser 202 and the second laser 204 within the device 200.

[0050] In some examples, if the first and second series of optical pulses are at slightly different but close frequencies (e.g., a few Hz apart), a beat frequency may occur between the first and second series of optical pulses (the beat frequency may make it difficult to distinguish the optical pulses). Therefore, the first frequency of the first series of optical pulses may have an intentionally introduced frequency offset relative to the second frequency of the second series of optical pulses in order to separate the first and second frequencies to avoid the beat frequency. The intentionally introduced frequency offset should be sufficient (e.g., at least a few GHz, such as 10 GHz) to avoid the beat frequency phenomenon. This may help to further reduce nonlinear optical damage.

[0051] Figure 2B201, which makes the first SOP of the first series of pulses and the second SOP of the second series of pulses received at the PBC 210 orthogonal to each other. In some examples, the PBC 210 includes two input ports, each of which receives a corresponding series of optical pulses from corresponding lasers 202, 204 through corresponding optical fibers 206, 208. The PBC 210 can be used to (set or otherwise be able to) combine two optical signals with two orthogonal polarizations. At each input port, only one polarization of the received signal is combined at the optical interface 228. As Figure 2B As shown in FIG. 1 , a first series of optical pulses is generated by a first laser 202 and received at a PBC 210 via a first optical fiber 206. The first optical fiber 206 can be rotated about its main axis 206(1) so that when the PCB 210 receives the first series of pulses, the first series of optical pulses are linearly polarized with one polarization (e.g., with X polarization). Similarly, a second series of optical pulses is generated by a second laser 204 and received at the PBC 210 via a second optical fiber 208. The second optical fiber 208 can be rotated about its main axis 208(1) so that when the PCB 210 receives the second series of pulses, the second series of optical pulses are linearly polarized with a second orthogonal polarization (e.g., with Y polarization). In this way, the first optical fiber is configured to have a first orientation and the second optical fiber is configured to have a second orientation, wherein the first orientation of the first optical fiber 206 is physically rotated 90° relative to the second orientation of the second optical fiber 208. Therefore, when the PCB 210 receives the SOP of the first series of optical pulses and the SOP of the second series of optical pulses, they are orthogonal to each other. The first and second series of optical pulses can then pass through the PBC 210 and be combined by the PBC 210 at the optical interface 228. The output of the PBC 210 is an unpolarized optical signal as a result of this combination. This physical rotation of the first and second optical fibers (e.g., a 90° relative rotation) can help to make the first and second series of optical pulses have orthogonal SOPs relative to each other when received at the PBC 210 without introducing additional hardware costs.

[0052] Figure 2B It is an illustrative example of utilizing the physical rotation of optical fibers 206, 208 to make the combined optical signal generated from PBC 210 have dual orthogonal polarizations. However, these examples are not intended to be limiting. In other examples, any other possible configuration in optical signal generator 201 can be applied to make PCB 210 receive the first and second series of pulses with SOPs orthogonal to each other, so that the combined optical signal generated from PBC 210 is a non-polarized signal with dual orthogonal polarizations. For example, other embodiments can introduce additional optical components (e.g., polarized light filters) into optical signal generator 201.

[0053] In some possible configurations, for example, Figure 2B As shown in , the optical signal generator 201 may further include a modulator 230. The modulator 230 may be an electro-absorption modulator (EAM), which may be used to modulate the intensity (e.g., power) of each pulse generated from the PBC 210 by applying a voltage. The modulator 230 and the voltage may be controlled by a control circuit 232. Since the modulator 230 is a polarization-insensitive modulator, the dual orthogonal polarizations of the unpolarized signal generated from the PBC 210 will not be affected by the modulation.

[0054] Figure 2C An alternative exemplary optional optical signal generator 201 is shown, which generates an unpolarized signal including two SOPs orthogonal to each other. In this example, the optical signal generator 201 includes a laser 202 and a dual polarization Mach Zenhder modulator (DPMZM) 238. Figure 2C As shown in FIG. 1 , DPMZM 238 includes first and second Mach-Zehnder modulators (MZMs) 234 ( 1 ), 234 ( 2 ) (generally referred to as MZM 234 ) and PBC 210 . Figure 2B Compared with the example of Figure 2C In the example of using a single laser 202, two MZMs 234 (1), 234 (2) can perform modulation and polarization adjustment or rotation on the corresponding received optical pulse series, and enable PBC 210 to receive two series of optical pulses with SOPs orthogonal to each other. For example, a series of optical pulses generated by laser 202 is divided into two channels, wherein the first and second separated series of optical pulses are sent respectively. (For example, by control circuit 236) control the first MZM 234 (1) to adjust the first separated series of optical pulses to align along a linear polarization (for example, with X polarization), and the second MZM 234 (2) is used to control the second separated series of optical pulses to have a polarization (for example, Y polarization) orthogonal to the first series. Since the two series of optical pulses received by PBC 210 are adjusted to have orthogonal SOPs, and the optical signal generated by PCB 210 includes dual orthogonal SOPs, the generated optical signal is non-polarized. In some examples, at least one of the first MZM 234(1) and the second MZM 234(2) may also be controlled to introduce a frequency offset between the first and second separate series of optical pulses such that orthogonality of the SOP may be achieved for the combined optical signal generated by the PCB 210.

[0055] Therefore, even if a single laser 202 is applied in the optical signal generator 201, the DPMZM 238 can be used in the optical signal generator 201 to make the optical pulses combined at the two input ports of the PBC 210 orthogonally polarized with respect to each other so as to generate an unpolarized optical signal. Figure 2C As shown in FIG. 2 , the modulation and / or polarization performed at the DPMZM 238 may be controlled by the control circuit 236 .

[0056] In some examples, for each optical pulse having a corresponding polarization (e.g., X or Y polarization), control circuit 236 may control or adjust at least one of a rising slope time on a corresponding rising edge or a falling slope time on a corresponding falling edge to be greater than zero to further mitigate nonlinear impairments (e.g., XPM).

[0057] Reference now Figure 2D , another optical signal generator 201 according to an alternative embodiment is shown. The optical signal generator 201 includes a laser 202 and a depolarizer 242. The depolarizer 242 is an optical device for scrambling the polarization of optical pulses. In some examples, the depolarizer 242 can output randomly polarized light regardless of its input, or produce a pseudo-random output polarization. That is, although the laser 202 can generate a series of optical pulses all having the same SOP, the depolarizer 242 receives the series of optical pulses and scrambles the SOP of the received optical pulses to generate an unpolarized signal with a randomly varying polarization. It should be noted that in some examples, the series of optical pulses generated by the laser 202 may include an SOP having a linear polarization or a circular polarization. In Figure 2D In the example of , with respect to mitigating nonlinear impairments (e.g., XPM), for each optical pulse generated by the laser 202, the optical signal generator 201 may further include a control circuit 240 to control at least one of a rising slope time on a corresponding rising edge or a falling slope time on a corresponding falling edge to be greater than zero. In some other examples, the control circuit 240 may be an internal setting of the laser 202 to adjust the rising edge and / or the falling edge.

[0058] It should be noted that although Figures 2B to 2D The optical signal generator 201 shown in has different configurations, but the optical signal generated by each optical signal generator 201 is a non-polarized signal (for example, by combining a pulse with two orthogonal SOPs, or by randomizing the SOP of the pulse). Therefore, the nonlinear optical damage (XPolM and / or XPM) caused by the polarization of the generated optical signal can be significantly reduced. It should be understood that the optical signal generator 201 can have any one of the above-mentioned configurations and its variants so that the optical signal generator 201 can output a non-polarized optical signal. Within the scope of the present invention, other embodiments of the optical signal generator 201 can be used to output a non-polarized optical signal.

[0059] In addition, the control circuits 232, 236, and 240 shown as respective components external to the modulator 230, the DPMZM 238, and the laser 202 are illustrative and not intended to be limiting. In other possible applications, the control circuits 232, 236, and 240 may be internal to the modulator 230, the DPMZM 238, and the laser 202, or may have any other suitable configuration.

[0060] Figure 4 An exemplary method 400 is shown that can be implemented at the above-described apparatus 200 to generate an unpolarized optical signal in order to mitigate nonlinear optical impairments (e.g., XPM and / or XPolM) that are closely dependent on the polarization of the generated optical signal. The method 400 includes:

[0061] At step 402, first and second trains of light pulses are generated. In some examples, each light pulse in the first train of light pulses has at least one of power, pulse shape, or timing equal to each light pulse in the second train of light pulses.

[0062] At step 404, first and second trains of light pulses are received. Figure 2B In the example of FIG. 1 , the first optical fiber 206 and the second optical fiber 208 are physically positioned to be orthogonal relative to each other so that the first and second SOPs of the respective first and second trains of pulses received at the PBC 210 are orthogonal to each other.

[0063] In step 406, the first and second series of optical pulses are combined to output an unpolarized optical signal. This step may be performed in a manner such as Figure 2A The outputted unpolarized optical signal can be used to test or characterize the optical communication network under test, such as the optical fiber in the optical communication network 100.

[0064] Reference again Figure 2A In some examples, the apparatus 200 may include one or more input / output (I / O) interfaces 222 that may be implemented to interface with one or more appropriate input devices and / or output devices. One or more of the input devices and / or output devices may be included as components of the apparatus 200, or one or more of the input devices and / or output devices may be external to the apparatus 200. In some examples, the I / O interface 222 may include a display that presents the time delay and / or length of the monitoring fiber calculated by the processor 218.

[0065] In some applications, the device 200 may include one or more memories 224, which may include volatile or non-volatile memory (e.g., flash memory, random access memory (RAM) and / or read-only memory (ROM)). The non-transitory memory 224 may store instructions for execution by the processor 218, for example, to perform the present invention. The memory 224 may include other software instructions, for example, for implementing an operating system and other applications / functions. In some examples, one or more data sets and / or modules may be provided by an external memory (e.g., an external drive in wired or wireless communication with the device 200), or may be provided by a transient or non-transitory computer-readable medium. Examples of non-transitory computer-readable media include RAM, ROM, erasable programmable ROM (EPROM), electrically erasable programmable ROM (EEPROM), flash memory, compact disc read-only memory (CD-ROM), or other portable memory.

[0066] There may be a bus 226 that provides communications between components of the device 200, including the processor 218, I / O interface 222, and memory 224. The bus 226 may be any suitable bus architecture including, for example, a memory bus, a peripheral bus, or a video bus.

[0067] In the present invention, a kind of exemplary optical device is described.Optical device generates non-polarized light signal by combination with first and second series of light pulses of mutually orthogonal polarization.Due to the first SOP of the first series of light pulses and the second SOP of the second series of light pulses orthogonal polarizations relative to each other, the light signal with dual polarization (for example, comprising the first and second orthogonal SOP) produced shows as being similar to non-polarized signal.

[0068] In some examples, to prevent the frequencies of the first and second trains of light pulses from being too close and causing a beat frequency, the frequencies of the first and second trains of light pulses have at least one frequency offset relative to each other.

[0069] In some applications, for at least one of the first and second series of optical pulses, the rising slope time on the rising edge and / or the falling slope time on the falling edge of each optical pulse can be controlled to be greater than zero, which can help reduce tracking errors used to recover the payload-carrying optical signal carried on the monitoring optical fiber.

[0070] The disclosed example optical devices may be implemented as an OTDR, which may help improve the performance and reliability (eg, accuracy, measurement range, and instrument resolution) of an OTDR used to characterize or troubleshoot a monitored optical fiber or optical communication network under test.

[0071] Although the present invention describes methods and processes by steps performed in a certain order, one or more steps in the methods and processes may be omitted or changed as appropriate. Where appropriate, one or more steps may be performed in an order other than the order described.

[0072] Although the present invention is at least partially described in terms of methods, it will be understood by those skilled in the art that the present invention is also directed to various components for performing at least some aspects and features of the methods, whether by hardware components, software, or any combination thereof. Accordingly, the technical solution of the present invention can be embodied in the form of a software product. Suitable software products can be stored in pre-recorded storage devices or other similar non-volatile or non-transient computer-readable media, including digital video disks (DVDs), CD-ROMs, universal serial bus (USB) flash drives, removable hard disks or other storage media, etc. The software product includes instructions tangibly stored thereon, which enable a processing device (e.g., an onboard processor, a personal computer, a server, or a network device) to perform examples of the methods disclosed herein.

[0073] Certain adaptations and modifications may be made to the described embodiments. Therefore, the embodiments discussed above are considered to be illustrative rather than restrictive. Although the present invention has been described with reference to the illustrative embodiments, this description is not intended to be interpreted in a limiting sense. Various modifications and combinations of the illustrative embodiments and other embodiments of the present invention will be apparent to those skilled in the art after reference to this description. Therefore, the appended claims are intended to cover any such modifications or embodiments.

Claims

1. A device in an optical network, It is characterized in that The device at least comprises: first and second lasers, the first laser for generating a first series of light pulses and the second laser for generating a second series of light pulses; and A polarization beam combiner PBC, used for receiving the first and second series of optical pulses, each optical pulse in the first series of optical pulses is received with a first polarization state SOP, each optical pulse in the second series of optical pulses is received with a second SOP, the first SOP and the second SOP are orthogonal to each other, and the PBC is further used for combining the first and second series of optical pulses and outputting a non-polarized optical signal; Wherein a first frequency of the first series of light pulses generated from the first laser has a frequency offset relative to a second frequency of the second series of light pulses generated from the second laser.

2. The device according to claim 1, It is characterized in that Each light pulse in at least one of the first and second series of light pulses has a corresponding rising edge and a corresponding falling edge, the first laser being used to adjust at least one of a rising slope time on the corresponding rising edge or a falling slope time on the corresponding falling edge to be greater than zero for the light pulses in the first series of light pulses; and The second laser is used to adjust at least one of a rising slope time on the corresponding rising edge or a falling slope time on the corresponding falling edge of the light pulses in the second series of light pulses to be greater than zero.

3. The device according to claim 1, It is characterized in that The device at least includes: and first and second optical fibers, the first optical fiber coupling the first laser to the PBC, the second optical fiber coupling the second laser to the PBC, wherein the first optical fiber is arranged such that the first optical fiber has a first orientation physically rotated 90° relative to a second orientation of the second optical fiber, such that the first SOP of each of the first series of optical pulses received at the PBC is orthogonal to the second SOP of each of the second series of optical pulses received at the PBC.

4. The device according to claim 1, It is characterized in that Each light pulse in the first series of light pulses has at least one of power, pulse shape, or timing equal to each light pulse in the second series of light pulses.

5. The device according to claim 1, It is characterized in that The device also includes: The optical circulator is used to send the non-polarized light signal to the monitoring optical fiber.

6. The device according to claim 5, It is characterized in that The device also includes: The photodetector PD is used to receive the reflected signal scattered back along the monitoring optical fiber and measure the power and time delay of the reflected signal.

7. The device according to any one of claims 1 to 6, It is characterized in that The device includes an optical time domain reflectometer OTDR.

8. The device according to any one of claims 1 to 6, It is characterized in that Each of the first and second SOPs includes a linear polarization.

9. A method for generating an optical signal in an optical network, It is characterized in that The method comprises: generating a first series of light pulses and a second series of light pulses; Receiving the first and second series of optical pulses, each optical pulse in the first series of optical pulses is received with a first polarization state SOP, and each optical pulse in the second series of optical pulses is received with a second SOP, the first SOP and the second SOP being orthogonal to each other; and combining the first and second series of optical pulses to output an unpolarized optical signal; The first frequency of the first series of light pulses has a frequency offset relative to the second frequency of the second series of light pulses.

10. The method according to claim 9, It is characterized in that Each optical pulse in at least one of the first and second series of optical pulses has a respective rising edge and a respective falling edge, the method further comprising: At least one of a rising slope time on the corresponding rising edge or a falling slope time on the corresponding falling edge is adjusted to be greater than zero.

11. The method according to claim 9, It is characterized in that The first series of light pulses are carried on a first optical fiber, and the first optical fiber is arranged so that the first optical fiber has a first orientation physically rotated 90° relative to a second orientation of a second optical fiber, so that the first SOP of each of the first series of light pulses is orthogonal to the second SOP of each of the second series of light pulses carried on the second optical fiber.

12. The method according to claim 9, It is characterized in that Each light pulse in the first series of light pulses has at least one of power, pulse shape, or timing equal to each light pulse in the second series of light pulses.

13. The method according to claim 9, It is characterized in that The method further comprises: The optical signal is sent to the monitoring optical fiber.

14. The method according to claim 13, It is characterized in that The method further comprises: receiving a reflected signal scattered back along the monitoring optical fiber; and The power and time delay of the reflected signal are measured.

15. The method according to any one of claims 9 to 14, It is characterized in that Each of the first and second SOPs includes a linear polarization.

16. The method according to any one of claims 9 to 14, It is characterized in that The method is implemented on an optical time domain reflectometer OTDR.

Images