WO2003076990A1

WO2003076990A1 - Optical module formed on a planar lightguide circuit including a waveguide optical amplifier

Info

Publication number: WO2003076990A1
Application number: PCT/US2003/006405
Authority: WO
Inventors: Sergey Frolov; Allan James Bruce; Joseph Shmulovich; Tek-Ming Shen; Attila Mekis
Original assignee: Inplane Photonics, Inc.
Priority date: 2002-03-04
Filing date: 2003-03-04
Publication date: 2003-09-18
Also published as: AU2003217871A1

Abstract

In accordance with the present invention, an integrated, optical loss-compensating module is provided. The module includes a substrate (208) for supporting a planar lightguide circuit “PLC” (210) and a dispersion compensating element “TDC” formed on a first part of the substrate. A PLC active waveguide is formed on a second part of the substrate for imparting gain to the optical signal. The active waveguide has a first port optically coupled to the dispersion compensating element through which the optical signal is communicated between the dispersion compensating element “TDC” and the active waveguide.

Description

OPTICAL MODULE FORMED ON A PLANAR LIGHTGUIDE CIRCUIT INCLUDING A WAVEGUIDE OPTICAL AMPLIFIER

Statement of Related Application

[0001] This application claims the benefit of priority to U.S. Provisional Patent Application 60/361,905, filed March 4, 2002, entitled "Application of Erbium-Doped Waveguide Amplifier and Dispersion Compensator and Pin on Planar Lightwave Circuit Platform."

Field of the Invention

[0002] The present invention relates generally to planar lightguide circuits, and more particularly to a planar lightguide circuit that incorporates a planar waveguide optical amplifier.

Background of the Invention

[0003] Optical modules based on planar lightguide circuits (PLCs) promise to reduce the cost and size of optical components while at the same time enhancing their functionality. PLCs, which employ planar optical integration to manufacture waveguide circuits on silicon wafers, use processing techniques similar to those used in the silicon microelectronics industry. Doped-silica waveguides are usually preferred because they have a number of attractive properties including low cost, low loss, stability, and compatibility for coupling to laser diodes, other waveguides, high NA fiber and standard fiber. Such a waveguide is fabricated on a carrier substrate, which typically comprises silicon or silica. The substrate serves as a mechanical support for the otherwise fragile waveguide and it can, if desired, also play the role of the bottom portion of the cladding. In addition, it can serve as a fixture to which input and output fibers are attached so as to optically couple cores of an input/output fiber to the cores of the waveguide. [0004] The PLC fabrication process begins by depositing a base or lower cladding layer of low index silica on the carrier substrate (assuming the substrate itself is not used as the cladding layer). A layer of doped glass, referred to as the core layer, with a refractive index higher than that of the lower cladding is then deposited on top of the lower cladding layer. The core layer is subsequently patterned or sculpted into structures required by the optical circuits using photolithographic techniques similar to those used in integrated circuit fabrication. Lastly, a top cladding layer of low refractive index is deposited to cover the patterned waveguide core. The difference in refractive index between the core and cladding layers ofthe waveguide is mostly determined by the material system and the fabrication process. In practice, different waveguide structures and systems are used for different types of functions and trade-offs are made in the core dimensions and the refractive index difference to optimize different aspects of optical performance.

[0005] One problem with an optical module - whether formed from discrete fiber components or on a planar lightguide circuit - is that it introduces optical loss into the path or system in which it is inserted. Moreover, as the number of integrated components increases, the overall optical loss of the module becomes an increasing problem. In addition, in some applications reduced power levels can even prevent the optical module from functioning in accordance with performance requirements such as bandwidth and sensitivity. For example, a hybrid integrated optical WDM transceiver module for use in a fiber-to-the-home (FTTH) communication system is shown in FIG. 1. The transceiver includes a PLC platform on which a 1.3 micron laser diode, a 1.5 micron laser diode, a 1.3 micron photodetector and a 1.5 micron photodiode are flip-chip mounted. The PLC platform, which is formed on a silicon substrate, includes a y-branch optical coupler and a y-branch optical splitter that is formed from planar waveguides. The planar waveguides consist of a high refractive index core that is embedded in under-cladding and over- cladding layers of a lower refractive index. The y-branch optical coupler and splitter perform a multiplexing/demultiplexing function at wavelengths of 1.3 and 1.5 microns , and enable a 1.3/1.5 micron waveguide, a 1.3 micron waveguide and a 1.5 micron waveguide to be arranged on the PLC platform. A terrace formed on the substrate has a sidewall in which the waveguides terminate and against which the laser diode and photodiode are situated for communicating optical signals between them and the waveguides.

[0006] The particular photodiode that is used in the hybrid integrated transceiver is generally either a PIN diode or an avalanche-photo diode (APD). Since a PIN diode has a greater bandwidth but lower sensitivity while an APD provides gain and hence better sensitivity, the choice of diode will depend on the sensitivity and bandwidth that are required for a given application. In practice, use of PIN diode is generally limited to low bit rate applications (e.g., less than 10 Gb/s) where the requirement of receiver sensitivity is not as demanding. On the other hand, at a bit rate of 10 Gb/s an APD is preferred. The APD provides gain and hence has better sensitivity than a PIN, and a bandwidth that is still marginally acceptable at this bit rate. Since the APD's bandwidth is limited by its gain-bandwidth product (i.e., it cannot provide both a large gain and a wide bandwidth at the same time) while the bandwidth of an APD is marginally acceptable at lOGb/s, it does not have adequate bandwidth and sensitivity for higher bit rate applications such as 40 Gb/s. In contrast, a PIN diode has the necessary bandwidth but does not have the required receiver sensitivity for most system applications at 10 Gb/s and 40 Gb/s. The problem can be solved by providing pre-amplifier gain in front ofthe PIN diode to counter balance the losses in the transceiver such as in the y-branches, and to improve the sensitivity ofthe receiver.

[0007] Another example of a lossless module of great importance is an optical add/drop module (OADM) for use in metropolitan area networks, which is shown in FIG. 2. The OADM is provided in a node where most ofthe traffic continues around the metro ring network while selected traffic is being added and dropped at the node. In this OADM, the traffic is demultiplexed into bands or individual wavelengths and selected wavelengths are dropped and added. The through-traffic and the add-wavelengths are then multiplexed together. Due to losses in the multiplexer and demultiplexer, and the varying power levels ofthe add-wavelengths, power adjustment of each wavelength is necessary. A narrow band amplifier can be used to adjust the power ofthe bands or wavelengths that are added and dropped and to adjust the power level ofthe through- channels. . In addition, amplification can be used before the demultiplexer as a preamplifier and after the multiplexer as a booster amplifier.

[0008] Accordingly, it would be desirable to provide an optical module based on a planar lightguide circuit in which optical gain or amplification is included and there is substantially no optical loss in the optical module.

Summary of the Invention

[0009] In accordance with the present invention, an integrated, optical loss- compensating module is provided. The module includes a substrate for supporting a planar lightguide circuit (PLC) and a dispersion compensating element formed on a first part of the substrate. A PLC active waveguide is formed on a second part of the substrate for imparting gain to the optical signal. The active waveguide has a first port optically coupled to the dispersion compensating element through which the optical signal is communicated between the dispersion compensating element and the active waveguide.

[0010] In accordance with another aspect ofthe invention, a PIN is formed on a third part of the substrate. The PTN is optically coupled to the dispersion compensating element.

[0011] In accordance with another aspect of the invention, a passive waveguide is formed on the substrate optically coupling the active waveguide to the dispersion compensating element.

[0012] In accordance with another aspect of the invention, a second passive waveguide is formed on the substrate optically coupling the PIN to the dispersion compensating element.

[0013] In accordance with another aspect of the invention, the PLC active waveguide is a multicomponent glass waveguide.

[0014] In accordance with another aspect ofthe invention, multicomponent glass waveguide is a rare-earth doped glass waveguide.

[0015] In accordance with another aspect of the invention, the rare-earth doped glass waveguide is an erbium doped waveguide.

[0016] In accordance with another aspect of the invention, the multicomponent glass waveguide is a multicomponent silica glass waveguide.

[0017] In accordance with another aspect ofthe invention, an integrated, optical loss-compensating module is provided that includes a substrate for supporting a planar lightguide circuit (PLC) and at least one signal generating source formed on a first part of the substrate. A PLC active waveguide is formed on a second part of the substrate for imparting gain to the optical signal. The active waveguide has a first port optically coupled to the optical component through which the optical signal is communicated between the signal generating source and the active waveguide.

[0018] In accordance with another aspect ofthe invention, the signal generating source may be a laser, a VCSEL, or an LED. [0019] In accordance with another aspect ofthe invention, a passive element is formed on the substrate. The passive element is selected from the group consisting of WDMs, couplers, splitters, combiners, taps, filters, attenuators and switches. [0020] In accordance with another aspect ofthe invention, an integrated, optical loss-compensating module is provided. The module includes a substiate for supporting a planar lightguide circuit (PLC) and at least one optical component processing an optical signal formed on a first part ofthe substiate. The optical component is a passive element selected from the group consisting of WDMs, couplers, splitters, combiners, taps, filters, attenuators and switches. A PLC active waveguide is formed on a second part ofthe substrate for imparting gain to the optical signal. The active waveguide has a first port optically coupled to the optical component through which the optical signal is communicated between the optical component and the active waveguide.

Brief Description of the Drawings

[0021] FIG. 1 shows a hybrid integrated optical WDM transceiver module for use in a fiber-to-the-home (FTTH) communication system.

[0022] FIG. 2 shows a functional diagram of a lossless OADM that may be used for metro applications.

[0023] FIG. 3 shows one embodiment ofthe present invention in which an optical waveguide amplifier is integrated on the same PLC platform as an optical receiver.

[0024] FIG. 4 shows a cross-section through the optical module in FIG. 3 taken along line A-A'.

[0025] FIG. 5 shows an optical receiver employing an Erbium doped fiber amplifier

(EDWA) as an optical pre-amplification stage for the PTN diode that includes a tap, monitor PIN diode, and an ASE filter to improve the sensitivity ofthe receiver.

[0026] FIG. 6 shows an optical waveguide amplifier integrated on the same platform with a tunable dispersion compensating (TDC) waveguide to form a loss-less module. A monitor tap and PTN diode are also fabricated in the same PLC platform.

[0027] FIG. 7 shows a 40 Gb/s optical receiver including an optical pre-amplifier, monitor tap and PIN diode, tunable dispersion compensating (TDC) waveguide, a PIN diode and the receiver electronic circuits, all in the same PLC platform Detailed Description

[0028] As a preliminary matter, it is worthy to note that any reference herein to "one embodiment" or "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment ofthe invention. The appearances ofthe phrase "in one embodiment" in various places in the specification are not necessarily all referring to the same embodiment.

[0029] The present inventors have recognized that the optical loss arising in a PLC optical module can be readily compensated for by integrating a waveguide optical amplifier into the module to provide a substantially loss-less device. Furthermore, by integrating a waveguide amplifier with the optical module, the device provides superior performance and a small form factor, which leads to a lower cost structure. Moreover, the integration of a waveguide amplifier allows the input power, output power, and noise figure of the device to better controlled and even standardized, thus simplifying the design of system architectures.

FIRST EMBODIMENT

[0030] FIG. 3 shows a first embodiment of the present invention in which the PLC optical module is an optical receiver. The optical receiver 200 includes a waveguide optical amplifier 202, a PTN photodetector 225 and an electronic circuitry unit 227, which are all integrated on a silicon substrate 208. Waveguide optical amplifier 202 includes a planar doped waveguide 210 in which the optical signal to be processed propagates. Planar waveguide 210 may be a single or multimode waveguide and is doped with an active element such as a rare-earth element, e.g., erbium. The waveguide 210 includes input port 238 and output port 239. The waveguide optical amplifier 202 also includes a coupling element 220 for coupling optical pump energy to waveguide 210. Coupling element 220 may be any integrated component that can convey the optical pump energy from a pump source 240 to the waveguide 310 and may include, for example, directional couplers, or multimode interference (MMI) filters. Coupling element 220 is configured to couple optical energy at the pump wavelength to the waveguide containing optical energy at the signal wavelength. Pump source 240 may be a single mode or multimode source of optical energy such as a laser diode, for example.

[0031] Photodetector 225 is mounted in close proximity to the output port 239 of planar waveguide 210. The photodetector 225 receives the amplified optical signal from the output port 239 and converts the optical signal to an electrical signal so that it can be processed by the electronic circuitry unit 227. Photodetector 225 and electionic circuitry unit 227 may be located on a terraced portion of substrate 208.

[0032] FIG. 4 shows a cross-section through planar waveguide amplifier section 310 taken along line A-A' in FIG. 3. As shown, planar waveguide 310 includes substrate 308, lower cladding layer 302, core layer 304, and upper cladding layer 306. Core layer 304 has an index of refraction greater than the index of refraction of lower and upper cladding layers 302 and 306 so that the optical energy is substantially confined to the core layer 304. Core layer 304 is doped with an active element such as a rare-earth element (e.g., erbium).

[0033] The optical receiver 200 shown in FIG. 3 may advantageously serve as a high bit rate, e.g., 40 Gb/s, receiver. This functionality can be achieved because by providing the waveguide optical amplifier as a preamplification stage for the PTN, the limited sensitivity of the PLN becomes less problematic. In fact, this receiver arrangement as shown in Fig. 5, includes an integrated amplified spontaneous emission (ASE) filter and can provide better sensitivity than a receiver that employs an APD while also providing a greater bandwidth than an APD.

SECOND EMBODIMENT [0034] In a second embodiment of the invention a planar waveguide optical amplifier is integrated on the same PLC platform as a dispersion compensating waveguide. As explained below, this device can be used at the receiver end ofthe system to compensate for the dispersion of the transmission fiber. The amplifier reduces or even eliminates the loss associated with the dispersion compensating waveguide. [0035] Chromatic dispersion, in which the index of refraction of the transmission medium is dependent on wavelength, is a phenomenon that has an adverse effect on the quality of a -transmitted optical signal. Chromatic dispersion in a transmission fiber causes the different wavelengths of a signal to undergo different phase shifts, resulting in spreading or broadening of the signal, which can give rise to transmission errors. The chromatic dispersion can be reduced by periodically introducing a dispersion compensating fiber (DCF) or module (DCM) along the transmission path. Because of the loss ofthe dispersion compensating fiber, the dispersion compensating fiber is placed in between stages of a multistage in-line fiber optical amplifier but external to, the in-line fiber optical amplifier. To provide proper dispersion compensation to every channel requires the dispersion compensating fiber or module to have a dispersion slope (as a function of wavelength) that is equal in magnitude but opposite in sign to the dispersion slope ofthe transmission fiber. In practice, there is generally a mismatch in the dispersion slope ofthe transmission fiber and the dispersion compensator. As a result, there will be some remnant dispersion in the signal at the receiving end, particularly at the long wavelength channels. This remnant dispersion places a limit on the total distance the WDM signal can be transmitted at a bit rate of 10 Gb/s. At a bit rate of 40 G/s, the remnant dispersion makes transmission of a WDM signal virtually impossible. One approach to address this situation is to provide separate dispersion compensators for each channel at the receiving end to remove the remnant dispersion. Unfortunately, dispersion compensating fiber is generally a relatively high loss component that is also very bulky, and therefore is not suitable for per channel application

[0036] In the present invention the dispersion compensating fiber is replaced with a dispersion compensating waveguide. Moreover, the dispersion compensating waveguide is preceded by (or follows) a waveguide optical amplifier that is formed on the same substiate. The resulting dispersion compensating module, which is shown in Fig. 6, can be used not only at the receiver end ofthe system, but also as the mid-stage of a fiber optical amplifier (e.g., an erbium doped fiber amplifier), which would otherwise require an external dispersion compensating fiber.

THIRD EMBODIMENT [0037] In a third embodiment ofthe invention a PTN diode is incorporated into the dispersion compensating module set forth in the second embodiment of the invention. This embodiment can be used as part ofthe receiver to serve as an integrated dispersion compensating preamplifier to provide signal conditioning. Similar to the first embodiment ofthe invention shown in FIG. 3, an electronic circuitry unit, i.e., an electrical front-end amplifier such as a trans-impedance amplifier is also integrated with the dispersion compensating waveguide, waveguide amplifier, and PIN diode.

[0038] In this embodiment of the invention the EDWA not only compensates for the loss of the dispersion compensating waveguide, but it also provides high gain, low noise amplification of the signal to in effect increase the sensitivity of the PTN receiver. This integrated receiver shown in FIG. 7 is bit rate transparent and is particularly useful for high bit rate applications (e.g., 10 Gb/s and above). For example, this device is particularly useful at bit rates of 40 Gb/s, where tight dispersion tolerance requirements make channel -by-channel dispersion compensation a necessity and the wide bandwidth requirements prevent an APD from being used.

FOURTH EMBODIMENT

[0039] In a fourth embodiment ofthe invention a planar waveguide optical amplifier is integrated on the same PLC platform as a signal source such as an edge-emitting semiconductor laser, vertical cavity surface emitting laser (VCSEL) or light emitting diode (LED). In this embodiment of the invention the waveguide amplifier serves as a booster amplifier that allows the use of a low power, and hence low cost, signal source to provide a source power comparable to or even higher than a more complex, single signal source. This relaxation ofthe requirement on the optical power level of the source allows additional functionality to be built into the device. For example, the signal source that is integrated with the waveguide amplifier may be a tunable VCSEL, which can provide a low cost, high power and tunable signal source for both long haul and Metro applications. [0040] The integrated signal source in accordance with the fourth embodiment ofthe invention may also be used in a variety of other ways. For example, the device may be used as a booster amplifier located after a multiplexer. In some cases the multiplexer may be integrated on the same platform as the signal source and waveguide amplifier. In the latter arrangement the gain ofthe waveguide amplifier can be adjusted to provide gain equalization, thereby eliminating the need for a variable optical attenuator for power adjustment. FIFTH EMBODIMENT

[0041] In a fifth embodiment ofthe invention a planar waveguide optical amplifier is integrated on the same PLC platform as a signal conditioning device. In this way the signal conditioning device can be inserted into the transmission path without loss, or even with gain. Examples of signal conditioning devices that may be integrated with the waveguide amplifier include, without limitation, a chromatic dispersion compensator, a polarization dispersion compensator, a band-pass filter, a high-pass filter, a low pass filter, a gain tilt filter, and an optical delay line.

OTHER EMBODIMENTS

[0042] In other embodiments ofthe invention a planar waveguide optical amplifier is integrated with a variety other components that can be directly fabricated on a PLC platform. These embodiments may or may not be further integrated with any ofthe aforementioned embodiments to provide a more highly integrated device. Examples of such components that can be fabricated with the waveguide amplifier include, without limitation, arrayed waveguide gratings (AWG) that serve as a multiplexer (MUX) or demultiplexer (DEMUX), variable optical attenuators (VOA), switches, amplified spontaneous emission (ASE) filters, band-pass filters, couplers, taps, and WDM combiners and separators. The following list sets forth the functionality that can be achieved with some of these embodiments of the invention.

[0043] 1. A waveguide amplifier may be integrated with a PIN diode and a tap to provide integrated power monitoring for signals before and after amplification. [0044] 2. A waveguide amplifier may be integrated with a multiplexer to provide gain and power equalization. Such a device functions as a VOA and a MUX (VMUX), but also provides gain.

[0045] 3. A waveguide amplifier may be integrated with a demultiplexer to provide amplification on a channel-by channel basis and can replace, at lower cost, a preamplifier that employs doped fiber. [0046] 4. A waveguide amplifier may be integrated with a demultiplexer to provide amplification and channel power equalization. Such a device functions as demultiplexer with a variable optical attenuator, but which in addition provides gain.

[0047] 5. A waveguide amplifier may be integrated with a combiner to provide a

2x2 cross switching function. The waveguide amplifier can function as a switch by turning the amplifier on or off, hence eliminating the need for a switch while also providing gain.

[0048] 6. A waveguide amplifier may be integrated with a filter to suppress amplified spontaneous emission (ASE) noise.

[0049] 7. A waveguide amplifier may be integrated with a tap and a multiplexer to serve as the mid-stage of a multi-stage EDFA.

[0050] 8. A waveguide amplifier may be integrated with a tap, which can be used for power balancing in and optical add-drop module (OADM).

[0051] 9. A waveguide amplifier may be integrated with a tap, which can be used for power balancing in an optical cross connect (OXC).

[0052] 10. A waveguide amplifier may be integrated with a dispersion compensating waveguide, which can be used in an OADM to compensate added channels that have excessive chromatic dispersion.

[0053] An important advantage of all ofthe aforementioned embodiments ofthe present invention is that a waveguide amplifier can be readily combined with any system element or elements to provide an optical module that has a specified and standardized input power, output power, and noise figure. As a result of this standardization, system architectures can be easily designed with predictable performance. Moreover, modules can be replaced and substituted with no difference in insertion loss and OSNR performance, and hence system performance. Accordingly, a system architecture that is designed using these standardized modules can provide much greater flexibility to customers. Such modules serve as building blocks, with standardized input power, output power and noise figure, to facilitate plug-and-play functionality in the system design process.

Claims

Claims;

1. An integrated, optical loss-compensating module, comprising: a substrate for supporting a planar lightguide circuit (PLC); a dispersion compensating element formed on a first part of the substiate; and a PLC active waveguide formed on a second part of the substrate for imparting gain to the optical signal, said active waveguide having a first port optically coupled to the dispersion compensating element through which the optical signal is communicated between the dispersion compensating element and the active waveguide.

2. The module of claim 1 further comprising a PTN formed on a third part of the substrate, said PTN being optically coupled to the dispersion compensating element.

3. The module of claim 1 further comprising a passive waveguide formed on the substrate optically coupling the active waveguide to the dispersion compensating element.

4. The module of claim 2 further comprising a second passive waveguide formed on the substrate optically coupling the PIN to the dispersion compensating element.

5. The module of claim 1 wherein said PLC active waveguide is a multicomponent glass waveguide.

6. The module of claim 5 wherein said multicomponent glass waveguide is a rare-earth doped glass waveguide.

7. The module of claim 6 wherein said rare-earth doped glass waveguide is an erbium doped waveguide.

8. The module of claim 5 wherein said multicomponent glass waveguide is a multicomponent silica glass waveguide.

9. The module of claim 1 wherein said substrate is silicon substrate.

10. The module of claim 8 wherein said substrate is silicon substiate.

11. An integrated, optical loss-compensating module, comprising: a substrate for supporting a planar lightguide circuit (PLC); at least one signal generating source formed on a first part of the substrate; and a PLC active waveguide formed on a second part of the substiate for imparting gain to the optical signal, said active waveguide having a first port optically coupled to the optical component through which the optical signal is communicated between the signal generating source and the active waveguide.

12. The module of claim 11 wherein said signal generating source is a laser.

13. The module of claim 11 wherein said signal generating source is a VCSEL

14. The module of claim 11 wherein said signal generating source is an LED.

15. The module of claim 12 wherein said laser is tunable.

16. The module of claim 13 wherein said VCSEL is tunable.

17. The module of claim 11 further comprising a passive element formed on said substrate, said passive element being selected from the group consisting of WDMs, couplers, splitters, combiners, taps, filters, attenuators and switches.

18. An integrated, optical loss-compensating module, comprising: a substiate for supporting a planar lightguide circuit (PLC); at least one optical component processing an optical signal formed on a first part ofthe substiate, said optical component being a passive element selected from the group consisting of WDMs, couplers, splitters, combiners, taps, filters, attenuators and switches; and a PLC active waveguide formed on a second part of the substrate for imparting gain to the optical signal, said active waveguide having a first port optically coupled to the optical component through which the optical signal is communicated between the optical component and the active waveguide.

19. An integrated, optical loss-compensating module, comprising: a substiate for supporting a planar lightguide circuit (PLC); at least one optical component processing an optical signal mounted on a first part of the substrate; and a PLC active waveguide formed on a second part ofthe substiate for imparting gain to the optical signal, said active waveguide having a first port optically coupled to the optical component through which the optical signal is communicated between the optical component and the active waveguide.

20. The module of claim 19 wherein the optical component includes a PTN.

21. The module of claim 19 wherein the optical component includes a laser.

22. The module of claim 19 wherein the optical component includes a VCSEL.

23. The module of claim 19 wherein the optical component includes a dispersion compensating element.

24. The module of claim 19 wherein the optical component includes at least one passive element selected from the group consisting of WDMs, couplers, splitters, combiners, taps, filters, attenuators and switches.