English:
[0001] Wave Division Multiplexing (WDM) technology has been widely used in optical fiber communications in order to increase the transmission capacity for point to point connection through a single optical fiber. Various conventional WDM laser technologies have been developed and widely deployed over the years. Solutions suitable for Dense WDM (DWDM) applications and with high speed modulation performance at 10G or above are generally desired for efficient use of optical fibers to deliver the overwhelmingly increasing data transmission demands, especially in long haul and metro markets. Fixed wavelength externally modulated distributed feedback (DFB) lasers, e.g., electro-absorption modulated lasers (EML), and wavelength tunable lasers are currently dominating the market deployment. Tunable lasers have gradually increased its share in the market with the advantage of being colorless for better inventory management. The sophisticated and costly fabrication process tunable lasers may potentially pose a limit to its continuing increase of market share.
[0002] WDM technology is not only desired for point to point transmission in long haul and metro markets but also desirable for multipoint to multipoint connections through a single fiber in aggregation, sometimes referred to as a WDM Passive optical network (WDM-PON) system. Both EML and tunable lasers are typically too expensive for WDM-PON systems. Cost effective colorless laser solutions are desired for WDM-PON applications and there has been substantial research and development efforts in the past pursuing such solutions. Among the earlier efforts, an Optical Network Unit with injection locked Fabry-Perot (FP) laser and reflective optical semiconductor amplifier (RSOA) based on a seed light coming from a high power broadband light source sliced by the Arrayed Waveguide Grating multiplexer/demultiplexer (AWG MUX/DEMUX) or from a continuous wave (CW) WDM light source have been proposed. Other options have been proposed including a self-seeding architecture that uses the light emission from the RSOA in the ONU as its own seed light by placing a partial reflective mirror at the output of the AWG MUX to feedback some of the light emission from the RSOA itself. This approach virtually forms an external cavity laser that has a cavity length as long as the length of the fiber that connects the ONU to the passive WDM MUX node of the WDM-PON system. Concern of stability for such a long external cavity laser has been studied using a Faraday rotator mirror (FRM) to stabilize the polarization through the long optical fiber. The self-seeding architecture greatly simplified the WDM-PON system since it limits the light seeding configuration to the ONU service area between the ONUs and passive node, which is beneficial for the seamless deployment of the WDM-PON service and facilitates the potential convergence between future WDM-PON and the existing time division multiplexing-PON (TDM-PON) architectures. Nonetheless, the relevant solutions proposed so far are not able to provide satisfactory transmission performance in speed and distance.
[0003] Systems, methods and apparatus are proposed for an external cavity FP laser structure and a WDM-PON architecture that incorporates such lasers to address the above issues and constraints that limit the transmission performance and enables the construction of a WDM-PON system for high speed multipoint connections across a single fiber with a length greater than, for example, 20km.
[0004] In some implementations, a structure is disclosed that includes a FP cavity semiconductor laser diode, e.g., a FP laser diode, whose optical output passes, optionally, through a Faraday rotator (FR) that rotates the laser polarization by substantially 45 degrees. The laser beam is then coupled into a fiber, e.g., a long optical fiber or polarization maintenance fiber, with a length that, for example, could range from several hundred meters to several kilometers, which connects to a WDM filter, e.g. a DWDM filter, whose optical output is partially reflected by a Faraday rotator mirror (FRM). The FRM is composed of a FR followed by a partially reflective mirror. The transmitted optical signal is used for signal transmission. In some implementations, only a partially reflective mirror is required (e.g., no coupler). The reflected optical signal then passes through the WDM filter and the fiber to reach the FR or the laser diode cavity if no FR is included.
[0005] The polarization of the beam reflected by a FRM remains orthogonal with the forward going optical signal at any location in between the FR and the FRM even though the polarization might be distorted when passing through the long and potentially strained fiber. The reflected beam then passes through the FR with its polarization being rotated by the FR before it is injected back into the FP laser cavity. The double passes through the FR and FRM result in a total rotation of the polarization of the reflected beam to 360 degrees which aligns well with the original laser polarization from the FP laser cavity. The FR, optical fiber, WDM filter, the FRM and the front facet of the FP cavity form an external resonant cavity in which the optical field, spectrally purified by the WDM filter, resonates within the external cavity and with the FP cavity, and locks the FP laser into one of the FP cavity modes that aligns with the WDM filter. This resonance relaxes the requirement of the loss budget in the external cavity to lock the FP laser.
[0006] Moreover, the implementation of double FR architecture stabilizes the polarization from the long external cavity and locks the wavelength of the FP to a greater extent than previous implementations. As a result, the optical gain in the FP laser cavity does not need to have low polarization dependence. Since the reflected beam is filtered by the WDM filter, the injection back to the FP laser cavity locks its lasing mode to one of the FP cavity modes which coincides with the wavelength window defined by the WDM filter and creates a single mode operation essential for high speed long distance transmission. Typically, the FP laser itself is designed to have reasonably low cavity loss and is operated well above threshold to give an acceptable relative intensity noise (RIN) level with well suppressed spontaneous emission and noise dominated by the mode partition among the multi-longitudinal lasing modes. In some implementations, the RIN is further reduced once the lasing mode is locked to a single mode by filtered external optical feedback injection. With this arrangement, an optical signal that is low noise and less sensitive to refection from the transmission system can be produced. In some implementations, the FP cavity mode is required to be aligned to the WDM filter center. This can be accomplished by controlling the FP cavity temperature. In some implementations, data transmission can be accomplished by directly modulating the current injection into the FP cavity.
[0007] Having the FP laser biased well above a threshold can enable high speed modulation, though, such direct current modulation may lead to wavelength chirping and limit the transmission distance due to dispersion in the optical fiber. In some implementations, to resolve this issue an external modulator (EM) is placed in between the FP cavity and the FR. To facilitate the WDM laser source covering a broad wavelength range, the EM can be a broadband modulator. A Mach-Zehnder interferometer (MZI) based intensity modulator can be used as the EM which is typically broadband with high speed modulation capability. A semiconductor optical amplifier (SOA) can also be used in some implementations enabling modulation of the current injection which modulates the optical gain.
[0008] In some implementations, the reflected light passing through the EM will be re-modulated which can potentially induce disturbance to the forward going signal modulation in both MZI and SOA modulator instantiations. In addition when using a SOA modulator, the reflected light potentially can affect the saturated optical gain and degrade the quality of the forward signal modulation. With the FP laser operating well above the threshold, the degradation of the forward going signal modulation quality caused by the re-modulation of the back injection light is typically small. In some implementations, further improvement can be achieved where the FR is placed in between the FP and the EM. In this architecture, the reflected beam passing through the EM will have a polarization 45 degrees off compared to the designed polarization of the EM, and thus reduces the effect of re-modulation of the reflected beam passing through the EM. In this configuration, the light injected back to the FP laser remains well aligned with the original polarization to maintain the wavelength locking performance.
[0009] In some implementations, the external cavity laser is incorporated into a WDM-PON architecture. In one example configuration an array waveguide grating (AWG) based WDM MUX (or a tunable DWDM filter) replaces the WDM filter in the original structure. The AWG MUX serves the WDM filter function in the external cavity laser structure with each input port of the AWG defining a different lasing wavelength for the FP laser that connects to the specific input port, and combines all the laser inputs together at the output port to facilitate single fiber transmission.
[0010] In some implementations, the self-seeding WDM-PON architecture is used for both the optical line termination (OLT) and optical network unit (ONU). To facilitate bidirectional transmission through a single optical fiber, the wavelengths for the OLT transmitters can be allocated in the L-band while the transmitting wavelengths for the ONUs can be allocated in the C-band. A Cyclic AWG can be used to accommodate the different wavelength bands for upstream and downstream MUX and DEMUX through a single optical fiber. WDM couplers can optionally be used in between the transmitter and FR to separate the L-band light in the downstream signal for signal receiving in the ONU transceivers, and to separate the C-band light in the upstream signal for signal receiving in the OLT transceivers. In some implementations, each transceiver unit includes the transmitter, the receiver, the WDM coupler and the FR, which can be integrated, for example, into a small form factor pluggable device along with electronics needed to operate the device with digital data and control interface.
[0011] Taking advantage of the nature of the laser architecture, the external cavity FP laser can be seamlessly added to the TDM-PON system as a solitary WDM laser with a single channel WDM filter, or as a group of ONUs using AWG. Since the laser characteristics does not depend on seed light coming from the system such as the OLT side, there is no performance compromise in the laser characteristics from adding it to the existing system.
[0012] The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.
[0013] FIG. 1 shows an example of an external cavity coupled FP laser structure with direct modulation.
[0014] FIG. 2 shows an example of an external cavity coupled FP laser structure with external modulation.
[0015] FIG. 3 shows an example of an external cavity coupled FP laser structure with the FR in between the FP laser and the external modulator.
[0016] FIG. 4 shows an example of a WDM-PON architecture including an AWG
[0017] FIG. 5 shows an example of a complete multipoint to multipoint connector through a single optical fiber based on a WDM-PON system.
[0018] FIG. 6 shows adding a single channel self-injection locked FP laser as a WDM laser into an existing TDM-PON system.
[0019] FIG. 7 shows an example for converging a group of self-seeding WDM-PON ONUs into an existing TDM-PON system.
[0020] FIG. 8 shows an example WDM laser using a photonic integrated circuit chip.
[0021] FIG. 9 shows an example WDM laser array.
[0022] Like reference numbers and designations in the various drawings indicate like elements.
[0023] Referring now to FIG. 1, an example of an external cavity FP laser structure 100 is shown that enables high performance WDM transmission at high speed over a long distance. In the implementation shown, the basic laser structure includes a FP laser cavity, e.g., FP laser diode 102, whose output first passes through a Faraday rotator (FR) 104 and is then coupled into an optical fiber 106 with a length ranging from, for example, several hundred meters to several kilometers. The Faraday rotator (FR) 104 rotates the laser polarization by 45 degrees. In some implementations, no FR is included in the structure. Similarly, the optical fiber can be replaced by a polarization maintenance fiber in some implementations.
[0024] The optical fiber 106 is coupled to a wave division multiplexing (WDM) filter 108, e.g., a dense WDM multiplexer, whose output optical signal is coupled through a splitter 1 10 and partially reflected by a Faraday rotator mirror (FRM) 112. The polarization of the reflected beam is rotated by 270 degrees after transmitting through and reflected by the FRM. The transmitted optical signal is provided as an output 114 and is used for signal transmission. In operation, the reflected laser signal passes through the WDM filter 108 and the FR 104 again and reaches back to the FP cavity at FP laser diode 102.
[0025] The double passes through the FR 104 and the FRM 112 result in a total polarization rotation of substantially 360 degrees for the reflected optical signal and enable it to align well with the original laser output from the FP laser cavity, e.g., from FP laser diode 102, that lies in the same polarization plane. This filtered reflection is injected back to the FP laser cavity and locks its lasing mode to one of the FP cavity modes which coincides with the wavelength window defined by the WDM filter 108 and creates a single mode operation essential for high speed long distance transmission. Data transmission can be enabled by directly modulating the current injection into the FP cavity. The optical gain in the FP laser cavity does not need to have low polarization dependence.
[0026] Using this type of architecture, the FR, fiber, WDM filter, the FRM, and the front facet of the FP cavity form an external resonant cavity optically coupled to the FP cavity. The optical field spectrally purified by the WDM filter resonates within the external cavity and with the FP cavity and locks the FP laser into one of the FP cavity modes that aligns with the WDM filter. This nature of resonance relaxes the requirement of the loss budget in the external cavity to lock the FP laser effectively. Moreover, the implementation of double FR architecture stabilizes the polarization from the long external cavity and locks the wavelength of FP to a great extent of previous implementations.
[0027] In some implementations, no coupler is included, and the output of the WDM multiplexer is provided directly to a partially reflective component. In some implementations, different ways can be used to implement the partial reflection by the FRM. For example, a partially reflective mirror can be used without using the splitter. In another example a two-port array waveguide (AWG) filter can be used in which one port is connected to a FRM while the other port is used for transmission.
[0028] In some alternative implementations, the FR can be removed. In this scheme, the polarization in the long cavity is still stabilized but the FP laser diode must have a reasonably low polarization dependent gain (PDG), not very low PDG, since double round-trip passes in the cavity are needed to recover the polarization. A reasonably low PDG will ensure the effective locking of the FP laser wavelength. This is still an improvement over conventional implementation in which the polarization state is not stabilized and as a result, the laser performance is very sensitive to PDG, so very low PDG is required.
[0029] In some other implementations, the fiber used is a polarization (PM) fiber and both the FR an FRM are removed from the structure. The PM fibers are used to stabilize the polarization in the long cavity.
[0030] In some implementations, the FP cavity mode of the FP laser diode is required to be aligned to the WDM filter, e.g., to the center frequency of the WDM filter, when, for example, they are not inherently aligned. In some implementations, alignment can be achieved by tuning either the FP cavity or the WDM filter. In some implementations, heating elements can be coupled to one or both of the FP laser diode or the WDM filter to accomplish this tuning.
[0031] Referring now to FIG. 2, in another implementation, an example self-injection locked FP laser structure 200 with external modulation is shown. An external modulator (EM) 202 is located between the FP laser diode 102 and the FR 104 to achieve higher transmission performance. In some implementations, the EM can be a semiconductor optical amplifier (SOA) or a Mach-Zehnder interferometer (MZI) based intensity modulator. The concern of modulation speed of a SOA being typically slow in the prior RSOA studies is resolved in the proposed architecture. This is attributed to the much higher power injection into the SOA from the FP laser that operates well above threshold, which reduces the carrier lifetime in the SOA and enables the higher speed modulation performance.
[0032] Referring now to FIG. 3, in another implementation, a second example of an external cavity FP laser structure 300 with external modulation is shown. In this example, the FR 104 can be placed in between the FP laser diode 102 and the EM 202. The placement of the FR 104 between the FP laser diode 102 and the EM 202 can reduce the re-modulation of the reflected beam passing through the EM 202. In this architecture, the reflected beam passing through the EM will have a polarization 45 degree off compared to the designed polarization of the EM, and reduces the effect of re-modulation of the reflected beam passing through the EM. Additionally, the light injected back to the FP laser remains well aligned with the original polarization to ensure the wavelength locking effectiveness. By contrast, in the example shown in FIG. 2, the reflected light passing through the EM will be re-modulated, which might induce disturbance to the forward going signal modulation in both MZI and SOA modulator cases. Also in the case of SOA modulator, the reflected light might affect the saturated optical gain and degrade the quality of the forward signal modulation.
[0033] With the FP laser operating well above threshold, the degradation of the forward going signal modulation quality caused by the re-modulation of the back injection light should be small since the forward going signal does not come from the regeneration process that amplifies the back injection light but comes directly from the FP laser itself and is much stronger than the back injection light, which is used for locking the FP laser mode. Nevertheless, further improvement may be achieved when using the architecture shown in FIG. 3.
[0034] Referring now to FIG. 4, in another implementation, an example architecture 400 is shown where the external cavity FP laser diode can be incorporated into a WDM architecture using a multiplexer, e.g., an array waveguide grating (AWG) based WDM MUX 402. The AWG MUX provides the WDM filter function in the external cavity laser structure with each input port of the AWG defining a different lasing wavelength for the FP laser that connect to the specific input port, and combines all the laser inputs together at the output port to facilitate single fiber transmission. In a conventional WDM-PON system, the distance between the WDM wavelength locking of the FP laser. As shown in FIG. 4, the architecture of FIG. 2 is provided where the DWDM multiplexer is replaced with the AWG MUX.
[0035] FIG. 5 shows an example of a complete multipoint to multipoint connector through a single optical fiber based on a WDM-PON system as described above. The wave division multiplexing passive optical network (WDM PON) includes one or more optical line termination (OLT) points 502 coupled to inputs of a first multiplexer 504, e.g., a first array waveguide grating (AWG) multiplexer (MUX). Each optical line termination point 502 includes a FP laser diode, an external modulator (EM) that receives the output optical signal from the FP laser diode and provides the output optical signal as an input to a Faraday rotator. The output of the Faraday rotator can be provided as an input to the first multiplexer 504, e.g., a first AWG MUX. Each optical line termination can also include a receiver optical subassembly (ROSA) as shown. Multiple OLT's 502 can be included in the WDM-PON system, each providing an input to the first multiplexer 504.
[0036] The WDM-PON system further includes a first splitter 506 for receiving the optical signal from an output of the first multiplexer 504 and a first Faraday rotator mirror (FRM) 508 coupled to one port of the first splitter 506. As discussed above, a reflected optical signal is reflected back into the FR and EM prior to being injected into the FP laser cavity of the OLT. The second port of the first splitter couples the output optical signal from the OLT to an optical fiber 510 whose other end is coupled to a second splitter 512. The second splitter 512 is configured similarly to the first splitter 506 and has one port coupled to a second Faraday rotator mirror 514 (to produce another reflected optical signal). The input of the second splitter 512 is coupled to an output of a second multiplexer 516, e.g., a second array waveguide grating AWG MUX. The inputs to the second multiplexer 516 are coupled respectively to one or more optical network units (ONU's) 518. In some implementations, each ONU 518 includes a similar structure to the OLT, though different wavelength of light can be used for the laser, including a FP laser diode, an external modulator (EM), and a Faraday rotator (FR). In operation, an optical output of a respective multiplexer is partially reflected by a respective Faraday rotator mirror (FRM) as discussed above.
[0037] A seamless convergence of the WDM laser and WDM-PON architectures with an existing TDM-PON system can be accomplished without compromising the laser characteristics. FIGs. 6 and 7 show example architectures for integration with an existing TDM-PON system.
[0038] FIG. 7 shows an example of converging a group of WDM-PON ONUs into an existing TDM-PON system. In yet another implementation, the external cavity FP laser diode can serve as a solitary WDM laser using a single channel DWDM filter.
[0039] For some applications, such as the optical line termination (OLT), multiple WDM lasers can all be placed in the same equipment. The construction of the external cavity coupled FP laser can be implemented without the long fiber in between the FP and the WDM filter.
[0040] In some implementations, to implement the WDM laser, a FP laser is coupled to a so called photonic integrated circuit (PIC) chip, which has multiple optical elements integrated on the same substrate. FIG. 8 shows an example WDM laser using a photonic integrated circuit chip.
[0041] In such implementations, the PIC chip can contain, sequentially, an EM, a WDM filter, an optical splitter with one arm coupled to a reflector and the other arm coupled to an output end of the PIC chip. All the elements on the PIC chip can be coupled to each other with one or more integrated optical waveguides on the same substrate, which on one end of the PIC, is used to couple to the FP laser, and on the other end, is used to couple to an optical fiber for output applications. Since all elements are integrated on the same chip, polarization is well preserved. In some alternative implementations, to avoid the complexity of the PIC, the splitter and reflector can be moved out of the PIC and placed in the output fiber using, e.g., a short length PM fiber.
[0042] FIG. 9 shows an example WDM laser array. Since multiple WDM lasers can all be placed in the same OLT equipment, a compact and high density integrated WDM laser array can be implemented by coupling an array of FP lasers to a PIC that contains an array of optical waveguides coupled to an array of EMs on the same PIC substrate. The array of EMs are then coupled to, and are multiplexed by, a WDM MUX, such as AWG, into one waveguide coupled to an optical splitter with one arm coupled to a reflector and the other are coupled to the output end of the same PIC.
[0043] While reference is made above to specific structures and apparatus, aspects of the technology may be embodied in one or more methods. In one example method, an optical output from a FP laser diode is provided. The output optical signal is coupled via a fiber (optical or PM fiber) to an optical multiplexer where it is multiplexed to produce a multiplexed signal. The multiplexed signal is optionally split producing a first split signal. The multiplexed signal (or first split signal) is reflected back to a FP laser cavity associated with the FP laser diode. The method includes aligning one FP cavity mode of the FP laser diode with an optical pass band center generated by the optical multiplexer, and locking a FP laser wavelength to single mode operation. The method further includes controlling a polarization of light propagating through the fiber and reflected back to the FP laser diode to be aligned with a polarization of the FP laser diode.

中文:
[0001]波分复用(WDM)技术已经广泛应用于光纤通信,以便增加通过单个光纤的点到点连接的传输容量。多年以来,各种常规WDM激光器技术已经得到开发并广泛地得以部署。为了高效利用光纤来传递势不可挡地增长的数据传输需求——特别是在长距(long haul)和城域(metro)市场中,适合于密集WDM(DWDM)应用并具有10G或以上的高速调制性能的方案通常是所期望的。固定波长外部调制的分布式反馈(DFB)激光器(例如电吸收调制激光器(EML)和波长可调谐激光器)目前主宰着市场部署。可调谐激光器以对于更好的库存管理(inventory management)是无色的优点,在市场上逐渐增加了其份额。复杂且昂贵的制造工艺可能对可调谐激光器的市场份额的持续上升构成限制。
[0002]WDM技术不仅对于长距和城域市场中的点到点传输是所期望的,对于通过单个光纤聚合的多点到多点连接——有时称为WDM无源光网络(WDM-PON)系统——也是所期望的。EML和可调谐激光器两者通常而言对于WDM-PON系统太过昂贵。具有成本效率的无色激光器方案对于WDM-PON应用是所期望的,并且在过去进行了相当多的研究和开发工作寻求这样的方案。在先前的努力中,已经提出了具有注入锁定的法布里-珀罗(FP)激光器和反射光学半导体放大器(RSOA)的光网络单元,其基于来自于由阵列波导光栅复用器/去复用器(AWG MUX/DEMUX)切割的高功率宽带光源或者来自于连续波(CW)WDM光源的种子光。已经提出了其它选项,包括自接种(self-seeding)架构,其通过将部分反射的反射镜放置在AWG MUX的输出端以反馈来自RSOA本身的一些光发射来使用来自ONU中的RSOA的光发射作为其自身的种子光。这种方法实际上形成了外腔式激光器,该激光器具有与连接ONU和WDM-PON系统的无源WDM MUX节点的光纤的长度一样长的腔体长度。对于这种长的外腔式激光器的稳定性的担忧已经通过使用法拉第旋转镜(FRM)进行了研究以稳定通过长光纤的偏振。自接种架构极大简化了WDM-PON系统,因为它将光接种(seeding)配置限制到ONU和无源节点之间的ONU服务区域,这有利于WDM-PON服务的无缝部署并便于未来的WDM-PON和现有的时分复用–PON(TDM-PON)架构之间的潜在的融合。尽管如此,到目前为止所提出的相关方案在速度和距离上不能提供满意的传输性能。
[0003]提出了用于外腔式FP激光器结构和WDM-PON架构的系统、方法和装置,该WDM-PON架构结合有外腔式FP激光器,以解决限制传输性能的上述问题和约束,并使得能够构造用于跨长度大于例如20km的单个光纤的高速多点连接的WDM-PON系统。
[0004]在一些实现中,公开了一种结构,该结构包括FP腔半导体激光器二极管(例如FP激光器二极管),FP腔半导体激光器二极管的光学输出可选地通过法拉第旋转器(FR),FR将激光偏振旋转大致45度。激光束接着耦合到长度范围例如可从几百米到几千米的光纤(例如,长的光纤或保偏光纤)中,该光纤连接到WDM滤波器(例如DWDM滤波器),该WDM滤波器的光学输出由法拉第旋转镜(Faraday rotator mirror,FRM)部分地反射。该FRM由跟有部分反射的反射镜的FR组成。所传输的光学信号被用于信号传输。在一些实现中,仅需要部分反射的反射镜(例如,没有耦合器)。被反射的光学信号接着通过WDM滤波器和光纤以到达FR或者到达激光器二极管腔(如果不包括FR的话)。
[0005]由FRM反射的激光束的偏振在FR和FRM之间的任何位置处保持与前向光学信号正交,即使在通过长且潜在应变的光纤时偏振可能失真。接着,在被注入回到FP激光器腔之前,被反射的激光束通过FR使其偏振被FR旋转。双重通过FR和FRM产生被反射激光束的偏振的360度的总旋转,其很好地与来自FP激光器腔的原始激光偏振对准。FR、光纤、WDM滤波器、FRM和FP腔的前端面形成外部谐振腔,其中由WDM滤波器光谱提纯的光场在外部腔体内与FP腔谐振,并将FP激光器锁定到FP腔模中与WDM滤波器对准的一个。这种谐振放宽了对外部腔体中锁定FP激光器的损耗预算的要求。
[0006]此外,双重FR架构的实现在比先前的实现更大的程度上稳定来自长外部腔体的偏振并锁定FP的波长。结果,FP激光腔中的光学增益不需要具有低的偏振依赖。由于被反射的激光束由WDM滤波器滤波,所以回到FP激光器腔的注入将其激射模(lasing mode)锁定到FP腔模中的一个腔模,该腔模与由WDM滤波器定义的波长窗口一致并创建对于高速长距离传输是重要的单模操作。典型地,FP激光器本身被设计为具有合理低的腔体损耗,并远高于阈值地操作以给出具有很好地抑制的自发发射和由多纵激射模之间的模分割支配的噪声的可接受的相对强度噪声(RIN)水平。在一些实现中,一旦激光模由经滤波的外部光学反馈注入被锁定到单个模,RIN被进一步降低。通过这种设置,可以产生低噪声并且对来自传输系统的反射较不敏感的光学信号。在一些实现中,需要将FP腔模与WDM滤波器中心对准。这可以通过控制FP腔体温度来实现。在一些实现中,数据传输可以通过直接调制到FP腔体中的电流注入来实现。
[0007]使FP激光器偏压得远高于阈值可以使能高速调制,尽管这种直接电流调制可能由于光纤中的色散而导致波长啁啾并限制传输距离。在一些实现中,为了解决这个问题,外部调制器(EM)被放置在FP腔体和FR之间。为了便于WDM激光源覆盖宽的波长范围,EM可以是宽带调制器。基于Mach-Zehnder(马赫-曾德尔)干涉仪(MZI)的强度调制器可以用作EM,其通常是宽带具有高速调制能力的。半导体光学放大器(SOA)也可以被用在一些实现中,使得能够进行调制光学增益的电流注入的调制。
[0008]在一些实现中,通过EM的被反射的光将被重新调制,这可以潜在地对MZI和SOA调制器实例两者中的前向信号调制引起干扰。另外,当采用SOA调制器时,被反射的光可以潜在地影响饱和光学增益并降低前向信号调制的质量。通过FP激光器远高于阈值地操作,由返回注入光的重新调制造成的前向信号质量的降低通常很小。在一些实现中,可以实现进一步的改进,其中FR放置在FP和EM之间。在这种架构中,通过EM的被反射的激光束将具有相比于EM的设计偏振偏离45度的偏振,从而降低了通过EM的被反射的激光束的重新调制的作用。在这种配置中,注入回到FP激光器的光保持与原始偏振很好地对准,以保持波长锁定性能。
[0009]在一些实现中,外腔式激光器被结合到WDM-PON架构中。在一种示例配置中,基于阵列波导光栅(AWG)的WDM MUX(或可调谐DWDM滤波器)代替原始结构中的WDM滤波器。AWG MUX起到外腔式激光器结构中的WDM滤波器功能,其中AWG的每个输入端口定义连接到该特定输入端口的FP激光器的不同的激射波长,并且AWG MUX在输出端口处将所有激光器输入组合在一起,以便于单个光纤传输。
[0010]在一些实现中,自接种WDM-PON架构用于光线路终结(OLT)和光网络单元(ONU)两者。为了便于通过单个光纤的双向传输,用于OLT发射器的波长可以被分配在L波段(L-band)中,同时用于ONU的传输波长可以被分配在C波段(C-band)中。循环AWG可被用于适应用于通过单个光纤的上行和下行MUX及DEMUX的不同的波长波段。WDM耦合器可以可选地用在发射器和FR之间,以对于ONU收发器中的信号接收分离下行信号中的L波段光,以及对于OLT收发器中的信号接收分离上行信号中的C波段光。在一些实现中,每个收发器单元包括发射器、接收器、WDM耦合器和FR,它们可以与电子器件一起被集成到例如小形状因数可插拔设备中,所述电子器件是以数字数据和控制接口来操作该设备所需的。
[0011]利用激光器架构的性质,可以将外腔式FP激光器无缝地添加到TDM-PON系统中,作为具有单通道WDM滤波器的单独WDM激光器或者作为使用AWG的一组ONU。由于激光器特性不依赖于来自系统(诸如OLT侧)的种子光,所以不存在由将其添加到现有系统而引起的激光器特性的性能折中。
[0012]本发明的一个或多个实施例的细节在以下说明和附图中进行阐述。从说明和附图以及从权利要求,本发明的其它特征、目的和优点将变得明晰。
[0013]图1示出了具有直接调制的外部腔体耦合FP激光器结构的示例。
[0014]图2示出了具有外部调制的外部腔体耦合FP激光器结构的示例。
[0015]图3示出了在FP激光器和外部调制器之间具有FR的外部腔体耦合FP激光器结构的示例。
[0016]图4示出了包括AWG MUX/DEMUX的WDM-PON架构的示例。
[0017]图5示出了基于WDM-PON系统的通过单个光纤的完整的多点到多点连接器的示例。
[0018]图6示出了将作为WDM激光器的单通道自注入锁定的FP激光器添加到现有TDM-PON系统中。
[0019]图7示出了用于将一组自接种WDM-PON ONU融合到现有TDM-PON系统中的示例。
[0020]图8示出了使用光子集成电路芯片的示例性WDM激光器。
[0021]图9示出了示例性WDM激光器阵列。
[0022]相同的参考数字以及名称在不同的附图中表示相同的元件。
[0023]现在参考图1,示出了外腔式FP激光器结构100的示例,该结构使得能够在长距离上以高速实现高性能WDM传输。在所示的实现中,基本的激光器结构包括FP激光器腔,例如FP激光器二极管102,其输出首先通过法拉第旋转器(FR)104、并接着耦合到长度范围例如从几百米到几千米的光纤106中。法拉第旋转器(FR)104使激光偏振旋转45度。在一些实现中,该结构中不包括FR。类似地,在一些实现中,该光纤可以由保偏光纤代替。
[0024]光纤106耦合到波分复用(WDM)滤波器108,例如密集WDM复用器,其输出光学信号耦合通过分光器110并由法拉第旋转镜(FRM)112部分地反射。在传输通过FRM并由该FRM反射后,被反射激光束的偏振被旋转270度。所传输的光学信号被提供为输出114并且被用于信号传输。在操作中,被反射的激光信号再次通过WDM滤波器108和FR 104并到达回FP激光器二极管102处的FP腔。
[0025]双重通过FR 104和FRM 112对于被反射的光学信号产生大致360度的总偏振旋转,并使其能够很好地与位于同一偏振平面的来自FP激光器腔(例如来自FP激光器二极管102)的原始激光输出对准。该经滤波的反射被注入回到FP激光器腔并将其激射模锁定到FP腔模中与WDM滤波器108所定义的波长窗口相一致的一个,并创建对于高速长距离传输是重要的单模操作。数据传输可以通过直接调制到FP腔中的电流注入来使能。FP激光器腔中的光学增益不需要具有低的偏振依赖。
[0026]使用这种架构,FR、光纤、WDM滤波器、FRM和FP腔的前端面形成光学耦合到FP腔的外部谐振腔。由WDM滤波器光谱提纯的光场在外部腔体内与FP腔谐振,并将FP激光器锁定到FP腔模中与WDM滤波器对准的一个。谐振的这种性质放宽了对外部腔体中有效地锁定FP激光器的损耗预算的要求。而且,双重FR架构的实现稳定了来自长外部腔体的偏振并且在之前实现的很大程度上锁定FP的波长。
[0027]在一些实现中,不包括耦合器,并且WDM复用器的输出被直接提供到部分反射的组件。在一些实现中,可以使用不同的方式来实现FRM的部分反射。例如,可以使用部分反射的反射镜而不使用分光器。在另一个示例中,可以使用两端口阵列波导(AWG)滤波器,其中一个端口连接到FRM而另一个端口用于传输。
[0028]在一些另选的实现中,可以去除FR。在该方案中,长腔体中的偏振仍然是稳定的,但是FP激光器二极管必须具有合理低的偏振相关增益(PDG)但不是非常低的PDG,因为需要双重往返通过腔体来恢复偏振。合理低的PDG将确保有效锁定FP激光器波长。这相对于常规实现仍是改进,在常规实现中偏振状态不稳定,并且因此激光器性能对PDG非常敏感,所以需要非常低的PDG。
[0029]在一些其它实现中,所用的光纤是偏振(PM)光纤,FR和FRM两者从该结构被去除。PM光纤用于稳定长腔体中的偏振。
[0030]在一些实现中,要求FP激光器二极管的FP腔模与WDM滤波器对准(例如与WDM滤波器的中心频率对准)——例如在它们固有地不对准时。在一些实现中,可以通过调谐FP腔或WDM滤波器来实现对准。在一些实现中,加热元件可以耦合到FP激光器二极管和WDM滤波器中的一者或两者来完成该调谐。另选地,可以使用一个或多个热电冷却器(TEC)。
[0031]现在参考图2,在另一实现中,示出了示例的具有外部调制的自注入锁定的FP激光器结构200。外部调制器(EM)202位于FP激光器二极管102和FR 104之间以实现较高的传输性能。在一些实现中,EM可以是半导体光学放大器(SOA)或基于Mach-Zehnder干涉仪(MZI)的强度调制器。在所提出的架构中解决了现有RSOA研究中SOA的调制速度通常很慢的担忧。这归因于从远高于阈值地操作的FP激光器到SOA中的高得多的功率注入,这降低了SOA中的载流子寿命并使能更高速的调制性能。
[0032]现在参考图3,在另一实现中,示出了具有外部调制的外腔式FP激光器结构300的第二示例。在该示例中,FR 104可以放置在FP激光器二极管102和EM 202之间。FR 104在FP激光器二极管102和EM 202之间的放置可以减小通过EM 202的被反射激光束的重新调制。在此架构中,通过EM的被反射的激光束将具有相比于EM的设计偏振偏离45度的偏振,并降低通过EM的被反射激光束的重新调制的作用。另外,注入回到FP激光器的光保持与原始偏振很好地对准,以确保波长锁定的有效性。通过对比,在图2所示的示例中,通过EM的被反射的光将被重新调制,这在MZI和SOA调制器两者的情况下可能引起对前向信号调制的干扰。同样在SOA调制器的情况下,被反射的光可能影响饱和光学增益并降低前向信号调制的质量。
[0033]通过FP激光器远高于阈值地操作,由返回注入光的重新调制导致的前向信号调制质量的降低应当很小,因为前向信号不是来自于放大返回注入光的重新生成过程,而是直接来自于FP激光器本身,并且比被用于锁定FP激光器模的返回注入光强很多。然而,当使用图3所示的架构时,可以实现进一步的改进。
[0034]现在参考图4,在另一个实现中,示出了示例的架构400,其中通过使用复用器(例如基于阵列波导光栅(AWG)的WDM MUX402),外腔式FP激光器二极管可以结合到WDM架构中。AWG MUX在外腔式激光器结构中提供WDM滤波器功能,其中AWG的每个输入端口定义用于连接到该特定输入端口的FP激光器的不同的激射波长,并在输出端口处将所有激光器输入组合在一起,以便于单个光纤传输。在常规WDM-PON系统中,WDM MUX/DEMUX和光网络单元(ONU)用户之间的距离可以是在几百米到几千米的范围。双重FR架构的实现稳定了来自长的外部腔体的偏振,并且对于确保FP激光器的波长锁定的有效性是关键的。如图4所示,提供了图2的架构,其中用AWG MUX来代替DWDM复用器。
[0035]图5示出了基于上述WDM-PON系统的通过单个光纤的完整的多点到多点连接器的示例。波分复用无源光网络(WDM PON)包括耦合到第一复用器504(例如第一阵列波导光栅(AWG)复用器(MUX))的输入的一个或多个光线路终结(OLT)点502。每个光线路终结点502包括FP激光器二极管、外部调制器(EM),外部调制器(EM)从FP激光器二极管接收输出光学信号并提供输出光学信号作为到法拉第旋转器的输入。法拉第旋转器的输出可以被提供作为到第一复用器504(例如第一AWG MUX)的输入。每个光线路终结还可以包括所示的接收器光学组装件(ROSA)。多个OLT 502可以被包括在WDM-PON系统中,各自提供到第一复用器504的输入。
[0036]WDM-PON系统还包括用于从第一复用器504的输出接收光学信号的第一分光器506和耦合到第一分光器506的一个端口的第一法拉第旋转镜(FRM)508。如上所讨论的,被反射的光学信号在被注入到OLT的FP激光器腔中之前被反射回到FR和EM中。第一分光器的第二端口将来自OLT的输出光学信号耦合到光纤510,光纤510的另一端耦合到第二分光器512。第二分光器512类似于第一分光器506地被配置,并具有耦合到第二法拉第旋转镜514的一个端口(以产生另一个被反射的光学信号)。第二分光器512的输入耦合到第二复用器516(例如第二阵列波导光栅AWG MUX)的输出。第二复用器516的输入分别耦合到一个或多个光网络单元(ONU)518。在一些实现中,每个ONU 518包括类似于OLT的结构——尽管光的不同波长可用于激光器,包括FP激光器二极管、外部调制器(EM)和法拉第旋转器(FR)。在操作中,相应复用器的光学输出由相应的法拉第旋转镜(FRM)部分地反射,如上所讨论的。
[0037]可以实现WDM激光器和WDM-PON架构与现有TDM-PON系统的无缝融合而不需要折中激光器特性。图6和图7示出了用于与现有TDM-PON系统集成的示例架构。更具体地,图6示出了将单通道外腔式FP激光器作为WDM激光器添加到现有TDM-PON系统中。
[0038]图7示出了将一组WDM-PON ONU融合到现有TDM-PON系统中的示例。在又一个实现中,通过使用单通道DWDM滤波器,外腔式FP激光器二极管可以用作单独的WDM激光器。
[0039]对于诸如光线路终结(OLT)的某些应用,多个WDM激光器可以都放置在同一设备中。外部腔体耦合的FP激光器的构造可以被实现而无需FP与WDM滤波器之间的长的光纤。另外,不需要管理由长的应变光纤潜在引入的偏振失真。
[0040]在一些实现中,为了实现WDM激光器,FP激光器被耦合到所谓的光子集成电路(PIC)芯片,该芯片具有集成在同一基板上的多个光学元件。图8示出了使用光子集成电路芯片的示例的WDM激光器。
[0041]在这样的实现中,PIC芯片可依次包含EM、WDM滤波器和分光器,分光器的一个臂耦合到反射器,另一个臂耦合到PIC芯片的输出端。PIC芯片上的所有元件可以通过同一基板上的一个或多个集成光波导而相互耦合,这些光波导在PIC的一端上用来耦合到FP激光器,并且在另一端上用来耦合到用于输出应用的光纤。由于所有元件被集成在同一芯片上,偏振得以很好地保持。在一些另选的实现中,为了避免PIC的复杂性,分光器和反射器可以从PIC移出并通过使用例如短长度的PM光纤而被放置在输出光纤中。
[0042]图9示出了示例的WDM激光器阵列。由于多个WDM激光器可以都放置在同一OLT设备中,可以通过以下来实现紧凑且高密度的集成WDM激光器阵列:将FP激光器的阵列耦合到包含光波导阵列的PIC,这些光波导耦合到同一PIC基板上的EM的阵列。EM的阵列接着耦合到WDM MUX(诸如AWG)、并由该WDM MUX复用到耦合到分光器的一个波导中,其中该分光器的一个臂耦合到反射器,另一个臂耦合到同一PIC的输出端。
[0043]虽然上面是参照特定的结构和装置,但是本技术的各个方面可被具体实现为一个或多个方法。在一个示例方法中,提供了来自FP激光器二极管的光学输出。输出光学信号通过光纤(光纤或PM光纤)耦合到光学复用器,输出光学信号在光学复用器处被复用以产生经复用的信号。经复用的信号可选地被分离,产生第一分离信号。经复用的信号(或第一分离信号)被反射回到与FP激光器二极管关联的FP激光器腔。该方法包括:将FP激光器二极管的一个FP腔模与由光学复用器生成的光学通带中心对准,和将FP激光器波长锁定到单模操作。该方法进一步包括控制通过光纤传播并被反射回到FP激光器二极管的光的偏振以与FP激光器二极管的偏振对准。