WDM-PON system with optical wavelength alignment function

A PLC-based wavelength-tunable WDM-PON system with an optical wavelength alignment function, the WDM-PON system comprises: a PLC platform formed on a silicon substrate; a semiconductor chip comprising an active region generating light and a passive region located in front of the active region for vertically coupling the light generated in the active region; a planar lightwave circuit (PLC) waveguide; one portion of a PLC platform where the semiconductor chip is surface mounted; waveguide Bragg grating (WBG) formed at a predetermined location of the PLC waveguide; a directional coupler transferring an optical power by permitting the passive region to approach the PLC waveguide; a heater terminal, which is formed on the WBG; and a V-groove for attaching an optical fiber to another end of the PLC waveguide. Accordingly, a WDM-PON system having a function of realizing a cost-effective optical wavelength alignment can be provided.

BACKGROUND OF THE INVENTION

This application claims the priority of Korean Patent Applications No. 2003-73448, filed on Oct. 21, 2003 and No. 2004-72041, filed on Sep. 9, 2004, in the Korean Intellectual Property Office, the disclosures of which are incorporated herein in their entirety by reference.

1. Field of the Invention

The present invention relates to a wavelength division multiplexing passive optical network (WDM-PON) system, and more particularly, to a WDM-PON with an optical wavelength alignment function for maintaining optical communication links regardless of an ambient temperature change.

2. Description of the Related Art

The variation of optical characteristics of modules included in a WDM-PON caused by a temperature change must be essentially considered in the WDM-PON.

In particular, since optical devices in a subscriber premises are exposed to the variance of ambient temperature in a subscribers network and the ambient temperature is different according to places in which the optical devices are installed, if this environment is not considered properly, optical communication quality cannot be satisfied. Therefore, a method of cost-effective solution with which a good quality of optical communications can be maintained regardless of the variance of ambient temperature in a WDM-PON system has been suggested.

That is, when a conventional WDM-PON system is applied to an actual environment, a WDM-PON system maintaining stable optical communication channels regardless of the variance of ambient temperature is required. Accordingly, a structure of stable and cost-effective optical communication links is required.

SUMMARY OF THE INVENTION

The present invention provides a wavelength division multiplexing passive optical network (WDM-PON) system with an optical wavelength alignment function for maintaining a good quality of optical communication links regardless of a temperature change.

The present invention also provides wavelength-tunable laser diodes (LDs) whose output optical wavelengths are tunable so that light sources of an optical transmitter of an optical line terminal (OLT) can be adapted to wavelength variations generated by the variance of ambient temperature of a WDM multiplexer/demultiplexer (MUX/DMX) located near subscribers.

The present invention also provides wavelength-tunable optical power monitor (OPM)-LDs formed in an optical transmitter and an optical receiver of an OLT in order to sense wavelength variations generated by the variance of ambient temperature of a WDM MUX/DMX located near subscribers and properly manage the wavelength variations.

The present invention also provides an optical transmitter of an ONT whose output optical wavelength is matched to a wavelength of a WDM MUX/DMX located near subscribers according to the variance of ambient temperature without an additional light source.

According to an aspect of the present invention, there is provided a wavelength division multiplexing passive optical network (WDM-PON) system with an optical wavelength alignment function, the WDM-PON system comprising: an optical line terminal (OLT) composed of an optical transmitter, which is composed of an OLT-LD array generating optical wavelengths for data transmission and a first wavelength control circuit aligning wavelengths of downstream transmission channels against variations of ambient temperatures, and an optical receiver, which is composed of a photo diode (PD) array and a second wavelength control circuit aligning a wavelength of an upstream transmission line against variations of ambient temperatures, wherein the optical transmitter further comprises a first WDM multiplexer (MUX) multiplexing a plurality of optical wavelengths output from the OLT-LD array and the optical receiver further comprises a first WDM demultiplexer (DMX) receiving a multiplexed optical wavelength and dividing the input multiplexed optical wavelength into individual wavelengths; a plurality of optical network terminals (ONTs), each ONT comprising an optical receiver, which receives a downstream optical wavelength for data transmission transmitted from the optical transmitter of the OLT, and an optical transmitter in which a wavelength-tunable waveguide Bragg grating (WBG) is formed, which forms an external cavity laser (ECL) generating a wavelength-tunable optical wavelength by controlling a temperature applied to the wavelength-tunable WBG; a second WDM DMX, which is located in a main distribution frame (MDF) placed near the plurality of ONTs, divides multiplexed optical wavelengths transmitted from the first WDM MUX via optical fiber into individual optical wavelengths, connects each optical wavelength to a relevant ONT of the plurality of ONTs, and has a first OPM-reflection mirror (RM) port reflecting an optical wavelength transmitted from the first wavelength control circuit; and a second WDM MUX, which is located in the MDF placed near the plurality of ONTs, multiplexes a plurality of optical wavelengths output from optical transmitters of the plurality of ONTs, transmits the multiplexed optical wavelengths to the first WDM DMX of the OLT via optical fiber, has a second OPM-RM port reflecting an output optical wavelength of the OPM-LD transmitted from the second wavelength control circuit, and has a WDM-RM port reflecting optical wavelengths to the optical transmitters of the ONTs according to how an optical wavelength output from the optical transmitter of an ONT is mismatched to a corresponding pass band of the second WDM MUX.

According to another aspect of the present invention, there is provided a PLC-based wavelength-tunable WDM-PON system with an optical wavelength alignment function, the WDM-PON system comprising: a PLC platform; a semiconductor chip comprising an active region generating light and a passive region located in front of the active region using a directional coupling principle for vertically coupling the light generated in the active region to another waveguide; a planar lightwave circuit (PLC) waveguide is formed on the PLC platform, and in one portion of which a PLC platform semiconductor chip is surface mounted by a passive alignment method, and at a predetermined location of PLC waveguide a waveguide Bragg grating (WBG) is formed; a directional coupler transferring an optical power by permitting the passive region to approach the PLC waveguide; a heater terminal, which is formed on the WBG in order to control a temperature of the WBG; and a V-groove, which is formed on the PLC platform for attaching an optical fiber into the end of the PLC waveguide.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, the present invention will now be described more fully with reference to the accompanying drawings, in which embodiments of the invention are shown.

FIGS. 1A and 1Bare schematic block diagrams of a wavelength division multiplexing passive optical network (WDM-PON) system with an optical wavelength alignment function responding to a temperature change according to an embodiment of the present invention.

FIGS. 1A and 1Bshow configuration modules whose optical wavelengths are changed in response to a temperature change in a structure of the WDM-PON and a loop A, a loop B, and a loop C for matching the optical wavelengths of the configuration modules.

FIG. 1Ashows a downstream transmission line directing from a central office (CO) to a subscriber. The downstream transmission line includes an optical line terminal (OLT) optical transmission module100composed of an OLT-laser diode (OLT-LD) module102for data transmission and an optical power monitor (OPM)-LD104, which is added to align wavelengths of the downstream transmission channels in response to a temperature change, an OLT-WDM multiplexer (WMUX)110located in an OLT, a remote node (RN)-WDM demultiplexer (WDMX)120located near the subscriber, and an optical network terminal-photo detector (ONT-PD)130, which is a subscriber optical reception module.

Components whose center optical wavelengths are changed due to the temperature change in the downstream transmission line are the OLT transmission module100, the OLT-WMUX110, and the RN-WDMX120. Here, according to conventional installation of the PON, the RN-WDMX120is exposed to the variance of ambient temperature most severely, and an electric device for controlling the temperature is not equipped in the RN-WDMX120. Therefore, a wavelength change of the RN-WDMX120can be taken as the reference for the other components, and the OLT transmission module100and the OLT-WMUX110align their wavelengths with reference to the changed wavelength of the RN-WDMX120.

FIG. 1Bshows an upstream transmission line directing from the subscriber to the CO. The upstream transmission line includes an ONT-LD150, which is a subscriber optical transmission module, an RN-WMUX160located near the subscriber, an OLT-WDMX170located in the OLT, and an OPM-LD180, which is added to align a wavelength of the upstream transmission channels in response to a temperature change.

Components whose center optical wavelengths are changed due to the temperature change in the upstream transmission line are the ONT-LD150, the RN-WMUX160, the OLT-WDMX170, and the OPM-LD180. Here, like the downstream transmission line, the RN-WMUX160is exposed to the variance of ambient temperature most severely according to characteristics of the PON, and an electric device for controlling the temperature is not equipped in the RN-WMUX160. Therefore, a wavelength change of the RN-WMUX160is referred to the other components, and the ONT-LD150aligns its wavelength with reference to the changed wavelength of the RN-WMUX160. Also, in a separate way from the ONT-LD150, the OLT-WDMX170and the OPM-LD180align their wavelengths with reference to the changed wavelength of the RN-WMUX160.

The wavelength alignment of the downstream transmission line is performed through the loop A. That is, the variance of the center wavelength of the RN-WDMX120is sensed by emitting a beam for a wavelength change measurement from the OPM-LD104to the RN-WDMX120and measuring the intensity of the beam reflected at the RN-WDMX120, and the OLT transmission module100aligns its wavelength with reference to the sensed variance of the center wavelength of the RN-WDMX120.

The wavelength alignment of the upstream transmission line is independently performed through the loop B and the loop C. That is, the wavelength alignment of the ONT-LD150is performed through the loop C. A misalignment of the RN-WMUX160is sensed by outputting an optical wavelength for data transmission from the ONT-LD150to the RN-WMUX160and measuring the intensity of an optical signal reflected at the RN-WMUX160, and the ONT-LD150aligns its wavelength with reference to the changed wavelength of the RN-WMUX160. Also, the wavelength alignment of the OLT-WDMX170and the OPM-LD180is performed through the loop B. That is, a misalignment of the RN-WMUX160is sensed by emitting a beam for a wavelength change measurement from the OPM-LD180to the RN-WMUX160and measuring the intensity of the beam reflected at the RN-WMUX160, and the OLT-WDMX170and the OPM-LD180align their wavelengths with reference to the changed wavelength of the RN-WMUX160.

FIG. 2is a block diagram illustrating a logical structure of a WDM-PON system according to an embodiment of the present invention.

A basic operation of the WDM-PON system will now be described with reference toFIG. 2. An OLT200located at a CO includes an optical transmitter210of a multi-wavelength external cavity laser (ECL) array type and an optical receiver220of a PD array type.

An ONT260includes an optical transmitter280including an ECL based on a broadband-tunable waveguide Bragg grating (WBG) and an optical receiver (ONT-PD)270.

The optical transmitter210of the OLT200further includes an OLT-WMUX204multiplexing a plurality of optical wavelengths output from the ECL array, and the optical receiver220of the OLT200further includes an OLT-WDMX206demultiplexing a multiplexed optical wavelength input from a plurality of ONTs260into wavelengths.

A multiplexed wavelength output from the optical transmitter210of the OLT200is transmitted via an optical fiber, divided into individual wavelengths by an RN-WDMX240located near subscribers, and transferred to the ONTs260. Different optical wavelengths output from the ONTs260are multiplexed by an RN-WMUX250located near the subscribers and transferred to the optical receiver220of the OLT200via an optical fiber.

The ONT260includes the ECL of an individual component type including a Fabry Perot laser diode (FP-LD) and a WBG in a same way with the OLT200. The important feature of the ECL is to generate all the optical wavelengths used in the OLT optical transmitter and to be not related to a specific optical wavelength since a center wavelength of a WBG reflection band can vary in a wide optical wavelength range by a heater.

Referring toFIG. 2, the optical transmitter210of the OLT200basically includes an OLT-LD array202emitting beams at frequency spaces recommended by ITU-T. Multi-wavelength optical signals output from the OLT-LD array202are multiplexed by the OLT-WMUX204and transmitted via a downstream optical transmission line. Since the OLT-WMUX204receives n LD output optical wavelengths for data transmission from the OLT-LD array202and an optical wavelength from an OPM-LD212and multiplexes the (n+1) optical wavelengths, the OLT-WMUX204has a (n+1)×1 structure. An optical circulator214is located between the OPM-LD212and the OLT-WMUX204. The optical circulator214transfers to a PD2216an OPM beam returning after the OPM beam is output from the OPM-LD212and is reflected at an OPM-RM242of the RN-WDMX240. A wavelength control circuit (WCC)218receives electric powers of the PD2216and a monitor PD (mPD) included in the OPM-LD212and controls a wavelength of the OLT-WMUX204, wavelengths of the OLT-LD array202, and a wavelength of the OPM-LD212. The WCC218also monitors a connection status of the downstream optical transmission line. A reference number211indicates a wavelength control circuit.

The optical receiver220of the OLT200includes an OLT-PD array208composed of n PDs and the OLT-WDMX206. The OLT-WDMX206has a (n+1)×1 structure to divide an input multiplexed optical wavelength into n optical wavelengths and transmit a beam output from an OPM-LD222to the subscribers. An optical circulator224is located between the OPM-LD222and the OLT-WDMX206. The optical circulator224transfers to a PD2226an OPM beam returning after the OPM beam is output from the OPM-LD222and reflected at an OPM-RM252of the RN-WMUX250. A WCC228receives electric powers of the PD2226and a monitor PD (mPD) included in the OPM-LD222and controls wavelengths of the OLT-WDMX206and a wavelength of the OPM-LD222. The WCC228also monitors a connection status of the upstream optical transmission line. A reference number221indicates a wavelength control circuit.

Since the RN-WDMX240includes n output ports for dividing a multiplexed optical wavelength input from the optical transmitter210of the OLT200into individual optical wavelengths and transmitting the optical wavelengths to the ONTs260and a port connected to the OPM-RM242reflecting an optical wavelength input from the OPM-LD212, the RN-WDMX240has a (n+1)×1 structure.

Different optical wavelengths on which upstream data outputs from the ONTs260are loaded are multiplexed by the RN-WMUX250and transferred to the OLT-WDMX206via the upstream optical transmission line. An optical circulator292is located between an ONT-LD285and the RN-WMUX250. The optical circulator292transfers to a PD2294a beam returning after the beam is output from the ONT-LD285and is reflected at a WDM-RM254of the RN-WMUX250. A WCC296receives electric powers of the PD2294and a monitor PD (mPD) included in the ONT-LD285and controls a wavelength of the ONT-LD285.

Since the RN-WMUX250includes n input ports for receiving optical wavelengths output from n ONT-LDs285, the OPM-RM252reflecting an optical wavelength input from the OPM-LD222in the optical receiver220of the OLT200, and the WDM-RM254reflecting an optical power to a relevant ONT260according to how much optical wavelengths output from individual ONT-LDs285are mismatched to corresponding wavelengths of the RN-WMUX250, the RN-WMUX250has a (n+1)×2 structure.

Methods of aligning optical wavelengths in the entire network in response to the variance of ambient temperature will now be described in detail.

1) In a PON type network, it can be assumed that the RN-WMUX250is a passive device working without electricity and is exposed to the variance of ambient temperature. Since wavelengths (or pass bands) of the RN-WMUX250varies according to the variance of ambient temperature, an optical wavelength output from the ONT-LD285is generally mismatched to a center wavelength of the RN-WMUX250when one ONT260trys to communicate, and the intensity of a beam reflected at the WDM-RM254of the RN-WMUX250varies according to a degree of the mismatch. The reflection beam reflected at the WDM-RM254and input to the optical transmitter280of the ONT260can be generally regarded as the upstream optical signal output from the ONT260, which tried to communicate, since a probability that a plurality of ONTs260simultaneously start to communicate is extremely low and some of ONTs260under communication have already aligned their output optical wavelengths of ONT-LDs285with corresponding wavelengths of the RN-WMUX250. Therefore, the WCC296of the ONT260, which just starts to communicate, compares the intensity of a reflected optical power input from the PD2294with an emitted optical power input from the mPD included in the ONT-LD285, and an optical wavelength of the ONT260is aligned by matching the optical wavelength of the ONT-LD285to the wavelength of the RN-WMUX250according to the comparison results.

2) The WDM-RM254of the RN-WMUX250reflects an optical power toward the optical transmitter280of the ONT260in proportion to how much optical wavelengths input to the n input ports of the RN-WMUX250are mismatched to corresponding wavelengths of the RN-WMUX250. The RN-WMUX250is formed so that an OPM-LD optical signal input from the OLT200passes at a minimized level toward the optical transmitter280of the ONT260via the n input ports of the RN-WMUX250.

3) The OPM-LD212included in the optical transmitter210of the OLT200transmits an OPM beam via the OLT-WMUX204downward. This OPM beam is reflected at the OPM-RM242attached to the RN-WDMX240and returns to the optical transmitter210of the OLT200. At this time, the intensity of the reflection beam of the OPM-RM242is determined according to a degree of mismatch between a wavelength of the RN-WDMX240changed by the ambient temperature and a wavelength of the OPM-LD212, and the reflection beam is input to the PD2216via the optical circulator214. The WCC218compares an output signal of the mPD included in the OPM-LD212with an output signal of the PD2216and controls the OLT-WMUX204, the OLT-LD array202, and the OPM-LD212so that wavelengths of the OLT-WMUX204, wavelengths of the OLT-LD array202, and a wavelength of the OPM-LD212are aligned to the wavelengths of the RN-WDMX240.

Also, the WCC218manages a status of the downstream optical transmission line on the basis of the signals output from the mPD included in the OPM-LD212and the PD2216.

4) Similarly, the OPM-LD222included in the optical receiver220of the OLT200transmits an OPM beam to the RN-WMUX250via the OLT-WDMX206and the upstream optical transmission line. This OPM beam is reflected at the OPM-RM252attached to the RN-WMUX250and returns to the optical receiver220of the OLT200. At this time, the intensity of the reflection beam of the OPM-RM252is determined according to a degree of mismatch between a wavelength of the RN-WMUX250changed by the ambient temperature and a wavelength of the OPM-LD222, and the reflection beam is input to the PD2226via the optical circulator224. The WCC228compares an output signal of the mPD included in the OPM-LD222with an output signal of the PD2226and controls the OLT-WDMX206and the OPM-LD222so that wavelengths of the OLT-WDMX206and a wavelength of the OPM-LD222are aligned to the wavelengths of the RN-WMUX250.

Also, the WCC228manages a status of the upstream optical transmission line on the basis of the signals output from the mPD included in the OPM-LD222and the PD2226.

FIG. 3is a schematic diagram of the RN-WDMX240ofFIG. 2. The RN-WDMX240has a 1×(n+1) ports structure.

Referring toFIG. 3, it is assumed that a wavelength output from the OPM-LD212of the OLT200is λn+1. An OPM-RM370exists at an output port of an RN-WDMX340from which a λn+1signal is output and reflects the λn+1signal. The OPM-RM370may be realized by configuring a mirror at a terminal of a relevant port using dielectric multi-layer coating or metal coating. Also, the OPM-RM370may be realized by optically connecting a discrete component such as a Bragg-reflector or a bulk mirror.

An arrayed-waveguide grating or a WDM filter can be used for a structure of the RN-WDMX340.

FIG. 4is a schematic diagram of the RN-WMUX250ofFIG. 2. The RN-WMUX250is composed of a 2×(n+1) WDM MUX.

Referring toFIG. 4, it is assumed that a wavelength output from the OPM-LD222of the OLT200is λn+1and wavelengths output from the ONTs260are λ1, λ2, . . . , λn. OPM-RM410exists in a relevant port42n+1 of an RN-WMUX430from which a λn+1signal is output and reflects the λn+1signal. Also, a WDM-RM470exists at a terminal of a second optical fiber452through which a multiplexed wavelength is output and reflects all of λ1, λ2, . . . , λnsignals. A first optical fiber451through which the multiplexed wavelengths are output forms the upstream optical transmission line. Considering an optical loss, a least optical signal is transferred to the second optical fiber452by putting at least 1 diffraction order difference between the two optical fibers451and452when the RN-WMUX250is designed.

Here, a reflection mirror component, such as the OPM-RM410and the WDM-RM470, may be realized by configuring a mirror at a terminal of a relevant port using dielectric multi-layer coating or metal coating. Also, the reflection mirror component, such as the OPM-RM410and the WDM-RM470, may be realized by optically connecting a discrete component such as the Bragg-reflector or the bulk mirror.

The arrayed-waveguide grating or the WDM filter can be used for a structure of the RN-WMUX430.

FIGS. 5A and 5Bare side views of ECLs, wavelengths of which are tunable by changing temperature, based on a PLC.

Referring toFIG. 5A, unlike a conventional butt coupling method of directly coupling an active region524of an InP chip520generating light and a WBG555on a straight line, the active region524and the WBG555exist on different waveguides and are vertically coupled.FIG. 5Ashows an ECL structure of changing an output optical wavelength by changing a temperature of the WBG555using a thermo-optic effect.

The InP chip520includes the active region524and a passive region522located in front of the active region524to vertically couple a generated beam to another waveguide. A semiconductor (InP family), a polymer, a nitride substance, or a silica can be used for the passive waveguide526, and a channel waveguide, a ridge-loaded waveguide, or a rib-waveguide can be used as a waveguide structure.

The InP chip520is formed on an InP substrate and surface-mounted on a PLC platform using a low price passive alignment method such as a flip-chip bonding.

In order to make efficient optical coupling of a beam generated in the active region524to a PLC waveguide560, the InP waveguide526has a structure that the size of the Passive waveguide526is getting smaller away from the active region524, so called a down-tapered Passive waveguide structure528. Also, the passive region522is formed so that a beam generated in the active region524is transferred to the PLC waveguide located560beneath or lost to the outside before the beam reaches the end of the Passive waveguide526in order to prevent the beam from being reflected at the end of the Passive waveguide526and input to the active region524again.

The PLC waveguide560also has a down-tapered PLC waveguide structure562so that size of a transferred beam gets larger in order to increase coupling efficiency between the PLC waveguide and Passive waveguide, and also to prevent a beam reflected at the WBG555from being reflected at the opposite end of the PLC waveguide560. Since a phase-matching condition between the two waveguides526and560can be satisfied by properly using the down-tapered PLC waveguide structure562, an optical coupling efficiency between the two waveguides526and560can be increased.

A refractivity of the InP waveguide526is at most two times lager than a refractivity of the PLC waveguide560, which uses the silica or polymer substance. When a refractivity difference between the InP waveguide526and the PLC waveguide560is large, since a directional coupler transferring an optical power by permitting one waveguide to approach another waveguide cannot satisfy the phase-matching condition, an optical transfer effect is low. In order to solve this problem, a leaky-mode grating-assisted directional coupler (LM-GADC)545satisfying the phase-matching condition between two waveguides by carving gratings on a coupled surface between the Passive waveguide526and the PLC waveguide560is suggested as shown inFIG. 5A.

The WBG555is formed on the PLC waveguide560, and a heater terminal550is installed above the WBG555to control a temperature of the WBG555. The wavelength-tunable WBG555is formed by the temperature control using the heater550.

Referring toFIG. 5B,FIG. 5Billustrates an etched surface-mount coupling (ESMC) method of etching a portion of the InP chip520and forming the Passive waveguide526using a dielectric material525having a refractivity similar to a refractivity of a PLC. The other parts refers to the description ofFIG. 5A.

FIGS. 6A and 6Bare side views of an embodiment of an ONT-TOSA, a wavelength of which is tunable by changing temperature, based on a vertically coupled ECL.

FIG. 6Ashows an example using the LM-GADC method described inFIG. 5A, andFIG. 6Bshows an example using the ESMC method described inFIG. 5B.

Referring toFIGS. 6A and 6B, unlike a conventional butt coupling method of directly coupling an active region624of an InP chip620generating light and a WBG655on a straight line, the active region624and the WBG655exist on different waveguides and are vertically coupled. Accordingly, a degree of difficulty of fabrication can be lowered, and simplicity of the fabrication process can be improved.

An mPD610is installed at a location close to the InP chip620in order to monitor an optical power of light generated in the active region624. The WBG655is formed on a PLC waveguide660, and a heater650is installed above the WBG655to control a temperature of the WBG655. Temperature of the wavelength-tunable WBG655is controlled the heater650. A spot-size converter670for magnifying a beam size in the PLC waveguide660is formed at a location coupled to an optical fiber680in order to increase an optical coupling efficiency between the PLC waveguide660and the optical fiber680. The optical fiber680is installed by being passive-aligned into a V-groove685formed on the PLC. Accordingly, an alignment process is simplified.

The WBG655fabricated using a silica material waveguide can have an optical wavelength variation range of around 2 nm in response to the variance of its temperature of 200° C. However, as for using this as a WDM wavelength-tunable light source, it is uneconomical since the number of channels is too small (for example, two channels are generated at 200 GHz spacing). On the other hand, since the WBG655fabricated using a polymer material waveguide can have an optical wavelength variation range of at most 30 nm in response to the variance of ambient temperature of 200° C., it is economical in terms of the number of channels (for example, eighteen channels can be generated at 200 GHz spacing).

The other parts ofFIGS. 6A and 6Brefer to the description ofFIGS. 5A and 5B.

FIGS. 7A and 7Bare side views of another embodiment of an ONT-TOSA, a wavelength of which is tunable by changing temperature, based on a vertically coupled ECL.

Referring toFIG. 7A, a basic structure is equal to the structure ofFIG. 6Bexcept a phase control region727inserted for a fine tuning and a stable operation of a resonant wavelength.FIG. 7Ashows a side view of the ONT-TOSA in which the phase control region727is integrated in an InP chip720. Here, a phase control is performed by controlling an amount of a current supplied to the phase control region727.

Referring toFIG. 7B, a basic structure is equal to the structure ofFIG. 6Bexcept a phase control unit790, which is located at a PLC waveguide760, inserted for a fine tuning and a stable operation of an resonant wavelength. Here, a phase control is performed by using an electro-optic effect or a thermo-optic effect.

The other parts ofFIGS. 7A and 7Brefer to the description ofFIGS. 5A,5B and6B.

FIGS. 8A and 8Bare a top view and a side view of an embodiment of an OLT-TOSA, a wavelength of which is tunable by changing temperature, based on a vertically coupled ECL.

Referring toFIGS. 8A and 8B, an FP-LD array820′ forms an ECL array by aligning one to one to a WBG array having reflection bands with frequency spacings recommended by ITU-T. In each ECL, a center wavelength of a relevant WBG reflection band is set so that a WDM optical signal with a predetermined frequency spacing can be transmitted.

By the WBG array permitting a wavelength of each WBG855to independently vary according to a thermo-optic effect, a yield against a fabrication error of a WBG855is dramatically improved, and a wavelength alignment according to the variance of WBG temperature is possible.

Multi-wavelength optical signals output from the output ends of the ECL array are finally output by being wavelength-multiplexed by a WDM MUX895monolithically integrated on one PLC chip. An optical fiber pigtail process of the OLT-TOSA can be simplified to single pigtail by integrating the ECL array and the WDM MUX895on the same PLC. An arrayed-waveguide grating (AWG) and a WDM filter can be used for the WDM MUX895.

A semiconductor chip generating light is fabricated as the FP-LD array820′ bar as shown inFIGS. 8A and 8B. Here, if individual FP-LD chips are used for respective ECLs, a process time is much longer when a passive alignment using a flip-chip bonding method is performed for all the FP-LD chips, and the alignment of already bonded chips may be distracted when other chips are aligned. These problems are solved using the FP-LD array820′ chip bar shown inFIGS. 8A and 8B. An mPD810is also fabricated as an mPD array810′ and monitors an optical output power of each ECL.

FIGS. 9A and 9Bare a top view and a side view of another embodiment of an OLT-TOSA, a wavelength of which is tunable by changing temperature, based on a vertically coupled ECL.

A basic structure ofFIG. 9Ais equal to the structure ofFIG. 8Aexcept a phase control region927inserted for a fine tuning and a stable operation of a wavelength.

Referring toFIGS. 9A and 9B, a basic structure is equal to the structure ofFIGS. 8A and 8Bexcept a phase control region927inserted for a fine tuning and a stable single mode operation.FIG. 9Ashows a side view of the OLT-TOSA in which the phase control region927is integrated in an InP chip920. Here, a phase control is performed by controlling an amount of a current supplied to the phase control region927.

FIG. 10is a block diagram illustrating a logical structure of a WDM-PON according to another embodiment of the present invention.

Referring toFIG. 10, OSAs configuring a suggested WDM-PON are shown. There are four kinds of OSAs, such as an ONT-TOSA1085for an ONT1060, an OLT-TOSA1011for an OLT transmitter1010, an OLT-receiver OSA (ROSA)1021for an OLT receiver1020, and an RN-WMUX/WDMX1030located near subscribers.

If these four kinds of OSAs are fabricated with a PLC, since optical communication parts of the suggested WDM-PON system are configured with 4 chips, the suggested WDM-PON system has advantages in terms of a physical volume and mass production. As different things fromFIG. 2, the OLT-LD array202ofFIG. 2is substituted with an OLT-ECL array1012, and the ONT-LD285is substituted with an ECL1085. Also, the OPM-LDs212and222generating light for alignment of wavelengths of the optical transmitter210and of the optical receiver220inFIG. 2are substituted with OPM-ECLs1002and1002′ ofFIG. 10. The other parts refer to the description ofFIG. 2.

As described above, the present invention relates to a WDM-PON with an optical wavelength alignment function for maintaining optical communication links regardless of ambient temperature changes. Since a function of maintaining stable optical communication links regardless of ambient temperature changes is necessary when a WDM-PON is applied to an actual environment, a WDM-PON system having a function of realizing a cost-effective optical wavelength alignment according to an embodiment of the present invention can solve this problem.