| JP09023191 | ROTARY TYPE OPTICAL FIBER SWITCH |
| JP56024834 | CAR-USE CONTROL DEVICE USING OPTICAL FIBER |
| JP08191274 | OPTICAL COMMUNICATION ADAPTOR |
MUN, Sil-Gu (209 Ho 104 Dong Daewoo Apt, Sangyeok 2dong Buk-Gu, Daegu 702-012, KR)
MOON, Jung-Hyung (306 Ho Dadong Jungnammansion, Hakjang-dong Sasang-Gu, Pusan 617-020, KR)
CHOI, Ki-Man (78-1, Seoho-dong Dong-gu, Daegu 701-240, KR)
LEE, Hoon-Keun (1407 Ho 101 Dong World Merdian Apt, Backcheon-dong Kyungsan-si, Kyungsangbuk-do 712-786, KR)
LEE, Chang-Hee (102 Ho 110 Dong Hanwool Apt, Shinseong-dong Yuseong-gu, Daejeon 305-345, KR)
MUN, Sil-Gu (209 Ho 104 Dong Daewoo Apt, Sangyeok 2dong Buk-Gu, Daegu 702-012, KR)
MOON, Jung-Hyung (306 Ho Dadong Jungnammansion, Hakjang-dong Sasang-Gu, Pusan 617-020, KR)
CHOI, Ki-Man (78-1, Seoho-dong Dong-gu, Daegu 701-240, KR)
LEE, Hoon-Keun (1407 Ho 101 Dong World Merdian Apt, Backcheon-dong Kyungsan-si, Kyungsangbuk-do 712-786, KR)
Claims
[1] A wavelength division multiplexed-passive optical network (WDM-PON) comprising: a first AWG, being positioned at a central office (CO) or an optical line termination (OLT) and having n-numbered output ports, for multiplexing/ de-multiplexing a transmitting signal; n-numbered first optical transceivers (TRx)(OLTl to OLTn) being respectively connected to the first AWG; a second AWG, being positioned at a remote node (RN) and having n-numbered output ports, for multiplexing/de-multiplexing the transmitting signal; a single mode fiber (SMF) being connected between the first AWG and the second AWG and being used for transmitting the transmitting signal; n-numbered second optical transceivers (TRx)(ONTl to ONTn) being positioned at subscribers (ONTs) and being respectively connected to the second AWG; and single mode fibers for separate transmissions being connected between the second AWG and the n-numbered second optical transceivers (TRx)(ONTl to ONTn), wherein the first optical transceivers (TRx)(OLTl to OLTn) and the second optical transceivers (TRx)(ONTl to ONTn) comprise, respectively, a WDM filter into which optical signals having different wavelength bands split through the first AWG and the second AWG are inputted; an optical transmitter (Tx), being connected to the WDM filter, for transmitting the optical signals having different wavelength bands through the WDM filter; and a receiver (Rx) for receiving the optical signals having different wavelength bands through the WDM filter, wherein the optical transmitter (Tx) comprises a multi-mode laser diode having a multi-contact and oscillating in a multi-mode; a driver for driving the multi-mode laser diode; a first mixer for up-converting a data transmission signal into a transmission format having a specific frequency range, and wherein the optical receiver (Rx) comprises a photo diode (PD) for converting the optical signals into electric signals; an radio frequency (RF) amplifier for amplifying the converted electric signals; a second mixer for down-converting the amplified electric signals into an original transmission format; a low pass filter (LPF) for only passing a spectrum range corresponding to signal components being converted into an original data transmission signal; and a clock and data recovery (CDR) for signal processing of the signal components being filtered by the LPF and recovering the data transmission signal.
[2] The WDM-PON of claim 1, wherein the optical transmitter (Tx) further comprises a band pass filter (BPF), being connected between the driver and the first mixer, for limiting a spectrum band of a transmission format of the data transmission signal only to a required bandwidth.
[3] The WDM-PON of claim 1 or 2, wherein the multi-mode laser diode uses a low noise range over a mode partition noise spectrum as a transmission band of the data transmission signal, and further uses a transmission format with high- spectrum efficiency.
[4] The WDM-PON of claim 3, wherein the a transmission format with high- spectrum efficiency is any one of a quadrature phase shift keying (QPSK) format, 16-quadrature amplitude modulation (QAM) format, 64-QAM format and 256-QAM format.
[5] The WDM-PON of claim 1 or 2, wherein the driver includes a current control for controlling electric current of the multi-contact in order to match the multi-mode laser diode with a specified center wavelength.
[6] The WDM-PON of claim 3, wherein the driver includes a current control for controlling electric current of the multiple contact in order to match the multi- mode laser diode with a specified center wavelength.
[7] The WDM-PON of claim 4, wherein the driver includes a current control for controlling electric current of the multiple contact in order to match the multi- mode laser diode with a specified center wavelength.
[8] The WDM-PON of claim 1 or 2, wherein the PD is embodied by an avalanche photo diode (APD).
[9] The WDM-PON of claim 3, wherein the PD is embodied by an avalanche photo diode (APD).
[10] The WDM-PON of claim 4, wherein the PD is embodied by an avalanche photo diode (APD).
[11] The WDM-PON of claim 1 or 2, wherein the first mixer mixes the data transmission signal with a carrier frequency f 1 inputted from outside and up-converts the data transmission signal, and wherein the second mixer down-converts the amplified electric signals by using the carrier frequency f 1 and converts the down-converted electric signal into the original data transmission signal.
[12] The WDM-PON of claim 3, wherein the first mixer mixes the data transmission signal with a carrier frequency f 1 inputted from outside and up-converts the data transmission signal, and wherein the second mixer down-converts the amplified electric signals by using the carrier frequency f 1 and converts the down-converted electric signal into the original data transmission signal.
[13] The WDM-PON of claim 4, wherein the first mixer mixes the data transmission signal with a carrier frequency f 1 inputted from outside and up-converts the data transmission signal, and wherein the second mixer down-converts the amplified electric signals by using the carrier frequency f 1 and converts the down-converted electric signal into the original data transmission signal.
[14] The WDM-PON of claim 1 or 2, wherein in case that the first AWG and the second AWG have a channel spacing of 100GHz or less, respectively, the WDM-PON further comprises a certain optical amplifier at a front end of the respective WDM filter.
[15] The WDM-PON of claim 3, wherein in case that the first AWG and the second
AWG have a channel spacing of 100GHz or less, respectively, the WDM-PON further comprises a certain optical amplifier at a front end of the respective WDM filter.
[16] The WDM-PON of claim 4, wherein in case that the first AWG and the second
AWG have a channel spacing of 100GHz or less, respectively, the WDM-PON further comprises a certain optical amplifier at a front end of the respective WDM filter.
[17] A wavelength division multiplexed-passive optical network (WDM-PON) comprising: a first AWG, being positioned at a central office (CO) or an optical line termination (OLT) and having n-numbered output ports, for multiplexing/ de-multiplexing a transmitting signal; n-numbered first optical transceivers (TRx)(OLTl to OLTn) being respectively connected to the first AWG; a second AWG, being positioned at a remote node (RN) and having n-numbered output ports, for multiplexing/de-multiplexing the transmitting signal; a single mode fiber (SMF) being connected between the first AWG and the second AWG and being used for transmitting the transmitting signal; n-numbered second optical transceivers (TRx)(ONTl to ONTn) being positioned at subscribers (ONTs) and being respectively connected to the second AWG; and single mode fibers for separate transmissions being connected between the second AWG and the n-numbered second optical transceivers (TRx)(ONTl to
ONTn), wherein the first optical transceivers (TRx)(OLTl to OLTn) and the second optical transceivers (TRx)(ONTl to ONTn) comprise, respectively, a WDM filter into which optical signals having different wavelength bands split through the first AWG and the second AWG are inputted; an optical transmitter (Tx), being connected to the WDM filter, for transmitting the optical signals having different wavelength bands through the WDM filter; and a receiver (Rx) for receiving the optical signals having different wavelength bands through the WDM filter, wherein the optical transmitter (Tx) comprises a multi-mode laser diode having a multi-contact and oscillating in a multi-mode; a driver for driving the multi-mode laser diode; a first mixer for up-converting a data transmission signal into a transmission format having a specific frequency range, wherein the optical receiver (Rx) comprises an avalanche photo diode (APD) for converting the optical signals into electric signals; a second mixer for converting the amplified electric signals into an original transmission format; a low pass filter (LPF) for only passing a spectrum range corresponding to signal components being converted into an original data transmission signal; and a clock and data recovery (CDR) for signal processing of the signal components being filtered by the LPF and recovering the data transmission signal, and wherein the optical signals received at the APD is larger than minimum current required for the CDR. |
Description
A WAVELENGTH DIVISION MULTIPLEXED-PASSIVE OPTICAL NETWORK USING MULTI-MODE LASER DIODES
HAVING A MULTI-CONTACT
Technical Field
[1] The present invention relates to a wavelength division multiplexed-passive optical network (WDM-PON) which may be embodied at a low cost by using multi-mode laser diodes having a multi-contact, which are a low noise light source having a low noise characteristic within a specific frequency range.
[2] More specifically, the present invention relates to a WDM-PON which embodies an access network at a low cost without inserting an incoherent light source using a multi- mode laser diode having a multi-contact, which may solve an increase of a mode partition noise or a reduction of optical power being caused due to a shift of a center wavelength of a laser diode depending an ambient temperature, by using a range with a low noise within a mode partition noise spectrum as a data transmission band in order to reduce the influence of a mode partition noise which may occur when transmitting one or more modes from an output of a laser diode having a multi-contact and oscillating in a multi-mode by using a spectrum-slicing method, and by changing electric current being injected into a multi-contact.
Background Art
[3] A current access network uses a telephone line or a cable modem using a coaxial cable, and such a cable modem has significant limitations relating to a bandwidth to be provided for subscribers and a transmission distance as well. However, recently it is required to provide a bandwidth of 100Mb/s or more while guaranteeing a high quality of service (Q S) in order to provide the subscribers with a triple play service (TPS) where images, data and sounds are integrated. A WDM-PON is recognized an ultimate alternative as one method capable of providing the subscribers with a bandwidth with no limitation substantially while regardless of a protocol and a transmission speed to be used for such a TPS. In general, because a WDM-PON is required that an optical fiber, which is a transmission medium, should be constructed to be extended to the subscribers at each home, it is necessarily required to embody an optical transceiver module and system at a low cost in order to provide the subscribers with the WDM- PON at a low cost. One example of this kind of a light source of a low cost WDM- PON is disclosed in Korean Patent Application No. 10-1999-0059923 entitled "A low- cost WDM source with an incoherent light injected Fabry-Perot semiconductor laser diode" filed on December 12, 1999 by Chang-Hee, Lee, et al. and registered on
February 8, 2002 as Korean Patent No. 10-0325687(hereinafter referred to '"687 Patent"). A wavelength-locked Fabry-Perot laser diode (F-P LD) disclosed in '687 Patent by Chang-Hee, Lee, et al. suggested a method of using a multi-mode F-P LD as a WDM-PON light source by making an F-P LD oscillating in a multi-mode to oscillate in a single-mode and simultaneously by fixing an oscillating wavelength thereof into a wavelength of an incoherent light source so as to reduce a mode partition noise, by inserting a broadband light source (BLS) into the an F-P LD oscillating in a multi-mode from outside. In this case, an erbium-doped fiber amplifier (EDFA) which emits amplified spontaneous emission (ASE), a high-power light emitting diode (LED), a super luminescent diode (SLD), etc. may be used as a BLS to be injected into an F-P LD.
Disclosure of Invention Technical Problem
[4] In a WDM-PON according to prior art, a BLS to be inserted from outside into an F-P
LD for locking a wavelength is a high-priced light source and thus is a disadvantageous factor when embodying a WDM-PON at a low cost.
[5] Accordingly, a WDM-PON which can be embodied at a low cost without injecting a high-priced BLS is required. Technical Solution
[6] The present invention is designed to solve the prior art problems described above and to provide a WDM-PON which may be embodied at a low cost by using a multi-mode laser diode having a multi-contact, which is a low-noise light source having a low- noise characteristic at a specific frequency range, without injecting a BLS from outside.
[7] More specifically, the present invention is to provide a WDM-PON at a low cost by using a low noise range over a mode partition noise instead of injecting a BLS from outside in order to reduce a mode partition noise and by embodying a large capacity of an access network through using a transmission format with high-spectrum efficiency as well, by improving transmission efficiency, and by compensating an increase of the mode partition noise or an optical power loss, due to the discrepancy between a transmission wavelength of an arrayed waveguide grating (AWG) being used as a wavelength-division multiplexer/de-multiplexer and a center wavelength of the multi- mode laser diode having the multi-contact depending on a temperature change, through controlling injected electric current which runs in the multi-contact of the multi-mode laser diode.
[8] According to a first aspect of the present invention, the present invention provides a wavelength division multiplexed-passive optical network (WDM-PON) comprising: a
first AWG, being positioned at a central office (CO) or an optical line termination (OLT) and having n-numbered output ports, for multiplexing/de-multiplexing a transmitting signal; n-numbered first optical transceivers (TRx)(OLTl to OLTn) being respectively connected to the first AWG; a second AWG, being positioned at a remote node (RN) and having n-numbered output ports, for multiplexing/de-multiplexing the transmitting signal; a single mode fiber (SMF) being connected between the first AWG and the second AWG and being used for transmitting the transmitting signal; n- numbered second optical transceivers (TRx)(ONTl to ONTn) being positioned at subscribers (ONTs) and being respectively connected to the second AWG; and single mode fibers for separate transmissions being connected between the second AWG and the n-numbered second optical transceivers (TRx)(ONTl to ONTn), wherein the first optical transceivers (TRx)(OLTl to OLTn) and the second optical transceivers (TRx)(ONTl to ONTn) comprise, respectively, a WDM filter into which optical signals having different wavelength bands split through the first AWG and the second AWG are inputted; an optical transmitter (Tx), being connected to the WDM filter, for transmitting the optical signals having different wavelength bands through the WDM filter; and a receiver (Rx) for receiving the optical signals having different wavelength bands through the WDM filter, wherein the optical transmitter (Tx) comprises a multi- mode laser diode having a multi-contact and oscillating in a multi-mode; a driver for driving the multi-mode laser diode; a first mixer for up-converting a data transmission signal into a transmission format having a specific frequency range, and wherein the optical receiver (Rx) comprises a photo diode (PD) for converting the optical signals into electric signals; an radio frequency (RF) amplifier for amplifying the converted electric signals; a second mixer for down-converting the amplified electric signals into an original transmission format; a low pass filter (LPF) for only passing a spectrum range corresponding to signal components being converted into an original data transmission signal; and a clock and data recovery (CDR) for signal processing of the signal components being filtered by the LPF and recovering the data transmission signal.
[9] According to a second aspect of the present invention, the present invention provides a wavelength division multiplexed-passive optical network (WDM-PON) comprising: a first AWG, being positioned at a central office (CO) or an optical line termination (OLT) and having n-numbered output ports, for multiplexing/de-multiplexing a transmitting signal; n-numbered first optical transceivers (TRx)(OLTl to OLTn) being respectively connected to the first AWG; a second AWG, being positioned at a remote node (RN) and having n-numbered output ports, for multiplexing/de-multiplexing the transmitting signal; a single mode fiber (SMF) being connected between the first AWG and the second AWG and being used for transmitting the transmitting signal; n-
numbered second optical transceivers (TRx)(ONTl to ONTn) being positioned at subscribers (ONTs) and being respectively connected to the second AWG; and single mode fibers for separate transmissions being connected between the second AWG and the n-numbered second optical transceivers (TRx)(ONTl to ONTn), wherein the first optical transceivers (TRx)(OLTl to OLTn) and the second optical transceivers (TRx)(ONTl to ONTn) comprise, respectively, a WDM filter into which optical signals having different wavelength bands split through the first AWG and the second AWG are inputted; an optical transmitter (Tx), being connected to the WDM filter, for transmitting the optical signals having different wavelength bands through the WDM filter; and a receiver (Rx) for receiving the optical signals having different wavelength bands through the WDM filter, wherein the optical transmitter (Tx) comprises a multi- mode laser diode having a multi-contact and oscillating in a multi-mode; a driver for driving the multi-mode laser diode; a first mixer for up-converting a data transmission signal into a transmission format having a specific frequency range, wherein the optical receiver (Rx) comprises an avalanche photo diode (APD) for converting the optical signals into electric signals; a second mixer for converting the amplified electric signals into an original transmission format; a low pass filter (LPF) for only passing a spectrum range corresponding to signal components being converted into an original data transmission signal; and a clock and data recovery (CDR) for signal processing of the signal components being filtered by the LPF and recovering the data transmission signal, and wherein the optical signals received at the APD is larger than minimum current required for the CDR.
Advantageous Effects
[10] In a WDM-PON according to the present invention, it is possible to embody a
WDM-PON at a low cost by varying injected electric current of a multi-mode laser diode having a multi-contact without injecting an incoherent light source from outside.
[11] In addition, a WDM-PON according to the present invention may accomplish a highspeed transmission with lGb/s or more when the WDM-PON has enough optical power from an optical transmitter to an optical receiver, by using a transmission format having high-spectrum efficiency.
[12] Further features and advantages of the present invention can be obviously understood with reference to the accompanying drawings where same or similar reference numerals indicate same components. Brief Description of the Drawings
[13] Fig. 1 illustrates a view of a WDM-PON structure using a multi-mode laser diode having a multi-contact according to the present invention.
[14] Fig. 2 illustrates a view showing a spectrum of a multi-mode laser diode having two
contacts as an example of a multi-mode laser diode having a multi-contact and a spectrum of an AWG used for a multiplexer/de-multiplexer.
[15] Fig. 3 illustrates RIN (Relative Intensity Noise) values measured depending on injected electric current, when filtering a multi-mode laser diode having two contacts according to an embodiment of the present invention described by referring to Fig. 2 through an AWG described by referring to Fig. 2.
[16] Fig. 4 illustrates a view of a slicing loss (upper graph) and RIN values (lower graph) depending on a wavelength mismatch characteristic between a multi-mode laser diode having two contacts according to an embodiment of the present invention and an AWG.
[17] Fig. 5 illustrates a view of a BER (Bit Error Rate) characteristic depending on temperatures in a WDM-PON, in case of using a multi-mode laser diode having two contacts according to an embodiment of the present invention as an optical transmitter. Mode for the Invention
[18] Hereinafter, structures and functions of preferred embodiments in accordance with the present invention are described in more detail with reference to the appended drawings.
[19] Fig. 1 illustrates a view of a WDM-PON structure using a multi-mode laser diode having a multi-contact according to the present invention.
[20] Referring to Fig. 1, the present invention comprises a first AWG, being positioned at a central office (CO) or an optical line termination (OLT) and having n-numbered output ports, for multiplexing/de-multiplexing a transmitting signal; n-numbered first optical transceivers (TRx)(OLTl to OLTn) being respectively connected to the first AWG; a second AWG, being positioned at a remote node (RN) and having n- numbered output ports, for multiplexing/de-multiplexing the transmitting signal; a single mode fiber (SMF) being connected between the first AWG and the second AWG and being used for transmitting the transmitting signal; n-numbered second optical transceivers (TRx)(ONTl to ONTn) being positioned at subscribers (ONTs) and being respectively connected to the second AWG; and single mode fibers for separate transmissions being connected between the second AWG and the n-numbered second optical transceivers (TRx)(ONTl to ONTn). As illustrated in Fig. 1, the OLT includes a plurality of optical transceivers (TRx) which are respectively connected to the first AWG. The n-numbered first optical transceivers (TRx)(OLTl to OLTn) positioned at the OLT and the n-numbered second optical transceivers (TRx)(ONTl to ONTn) positioned at the subscribers (ONTs) respectively comprise a combination of a WDM filter into which optical signals having different wavelength bands split through the first AWG and the second AWG are inputted, and an optical transmitter (Tx), being
connected to the WDM filter, for transmitting the optical signals having different wavelength bands through the WDM filter and a receiver (Rx) for receiving the optical signals having different wavelength bands through the WDM filter. In addition, a low- noise light source having a low-noise characteristic is used as a light source of the optical transmitter (Tx). Multi-mode laser diodes having a multi-contact are used as a specific example of such a low-noise light source. The multi-mode laser diodes having a multi-contact are driven by an analog driver. The analog driver includes a current control for controlling electric current of the multi-contact in order to match the multi- mode laser diodes having a multi-contact with a specified center wavelength. As described above, the present invention accomplishes a large capacity of a network at a low cost and improves transmission efficiency further by employing a multi-mode laser diode having a multi-contact as a light source of an optical transmitter where the multi-mode laser diodes having a multiple contact uses a low-noise range among spectra having a relative intensity noise (RIN) as a transmission band and by using a transmission format having high-spectrum efficiency, instead of compensating a noise degradation characteristic of a system due to a mode partition noise by injecting a BLS from outside.
[21] For accomplishing the above, an embodiment of the present invention described above employs a first mixer in order to use a binary phase shift keying (BPSK) format as a non-return to zero (NRZ) data transmission signal, where the BPSK format is a transmission format using a high frequency band except a low frequency band of approximately 300 MHz or less which will be described later referring to Fig. 3. The first mixer up-converts the NRZ data transmission signal into a transmission format having a specific frequency range by mixing the NRZ data transmission signal with a carrier frequency f 1 a carrier frequency and then up-converting the NRZ data transmission signal. In an embodiment of the present invention, 750 MHz is used for the carrier frequency fl. Further, the optical transmitter (Tx) according to the present invention may comprise a band pass filter (BPF) between the analog driver and the first mixer in order to limit the frequency band of a transmission format of the NRZ data transmission signal only to a required bandwidth.
[22] In the meanwhile, the optical receiver (Rx) includes a photo diode (PD) for receiving an optical signal. Although it is described that a PD is used in an embodiment of the present invention, it will be fully understood by any skilled person in the art that an avalanche photo diode (APD) with superior reception sensitivity may be used in the present invention. The PD converts the received optical signal into an electric signal, and the converted electric signal passes a radio frequency (RF) amplifier. The RF amplifier amplifies the converted electric signal to a minimum electric signal required for a second mixer. After that, the second mixer down-converts the amplified electric
signal by using a carrier frequency f 1 which is the same as the carrier frequency used at the optical transmitter (Tx) and then converts the down-converted electric signal into the original transmission format having a specific baseband frequency band. After that, the signal having the down-converted baseband frequency band is filtered by a low pass filter (LPF) in a manner that only a necessary signal band passes the LPF. After that, the filtered signal is signal processed by a clock and data recovery (CDR) and then an originally transmitted NRZ data transmission signal is recovered. In case that an APD having superior reception sensitivity is used instead of the PD and the received signal is larger than minimum current required for the CDR, the RF amplifier for amplifying the received signal may not be used.
[23] Although an embodiment of the present invention describes a structure and a principle of a WDM-PON, it will be fully understood by any skilled person in the art that an embodiment of the present invention will be applicable to a general optical transmission system.
[24] Fig. 2 illustrates a view showing a spectrum of a multi-mode laser diode having two contacts as an example of a multi-mode laser diode having a multi-contact and a spectrum of an AWG used for a multiplexer/de-multiplexer.
[25] Referring to Fig. 2, a solid line indicates the spectrum of a multi-mode laser diode having two contacts being used for an embodiment of the present invention, while a dot line indicates the spectrum of an AWG used for a multiplexer/de-multiplexer. In an embodiment of the present invention, a pass band has a flat-top type and an AWG where a bandwidth of 3dB is approximately 0.9nm is used.
[26] Fig. 3 illustrates RIN (Relative Intensity Noise) values measured depending on injected electric current, when filtering a multi-mode laser diode having two contacts according to an embodiment of the present invention described by referring to Fig. 2 through an AWG described by referring to Fig. 2.
[27] Referring to Fig. 3, Fig. 3 illustrates respective RIN values for the case when operating a multi-mode laser diode having two contacts at driving currents with 1.6, 1.8 and 2.0 times of threshold current (Ith) of the multi-mode laser diode. As can be seen from Fig. 3, the RIN value is high at a low frequency band of approximately 300MHx or less, while the RIN value decreases as the frequency band increases. Therefore, as described above by referring to Fig. 1, it is possible to improve a degradation characteristic due to a mode partition noise even without injecting a BLS from outside, in case of using a BPSK transmission format where a high frequency band except a low frequency band of approximately 300 MHz or less is used for a transmission band. Although it is described that the present invention uses a BPSK transmission format as a transmission format with high-spectrum efficiency, it will be fully understood by any skilled person in the art that a quadrature phase shift keying
(QPSK) format, 16-quadrature amplitude modulation (QAM) format, 64-QAM format or 256-QAM format, etc., for example, may be applicable to the present invention as any other transmission format with high-spectrum efficiency.
[28] Fig. 4 illustrates a view of a slicing loss (upper graph) and RIN values (lower graph) depending on a wavelength mismatch characteristic between a multi-mode laser diode having two contacts according to an embodiment of the present invention and an AWG.
[29] The AWG being used for performing a multiplexing/de-multiplexing operation according to an embodiment of the present invention illustrated in Fig. 4 is an athermal-type one and has no change depending on temperatures, while, in case of a laser diode being used for an optical transmitter (Tx) of each OLT or each ONT illustrated in Fig. 1, the maximum wavelength of the laser diode may vary depending on ambient temperatures. It is generally known that the maximum gain value of a laser diode is shifted toward a longer wavelength at a rate of 0.5nm/°C. If the variation of temperature is large, the difference between the wavelength filtered by the AWG and the wavelength having the maximum gain of the laser diode is significantly increased. As a result, the output optical power of the laser diode experiences a big loss after being filtered by the AWG, and the output power noise characteristic after being filtered by the AWG also experiences degradation as well. Characteristics depending on the difference between the wavelength having the maximum gain of the used laser diode and the wavelength of the AWG are illustrated in Fig. 4. In the graphs of Fig. 4, the left vertical axis indicates a wavelength of the AWG - a wavelength having the maximum gain of the laser diode (i.e., the difference between a wavelength value of the AWG and a wavelength value having the maximum gain of the laser diode). In this case, the definition of a wavelength value having the maximum gain of a laser diode refers to GR-486-CORE, Section 5.1 defined by the International Standardization Organization (ISO) of Telcordia Technologies, US. A wavelength value having the maximum gain of a laser diode defined by such an ISO is given by the formula (1) as follows:
[30] Center Wavelength (λ nm) = (∑λ x P) / σP (1)
[31] In the above formular (1), λ indicates an i peak wavelength and P indicates i peak power.
[32] When using the formular (1) and considering optical power required from an optical transmitting end to an optical receiving end including SMFs, the RIN value is required to be -96.2dB/Hz or less in order to transmit data at a speed of 155Mbps in an embodiment of the present invention illustrated in Fig. 4. In this case, a power budget between the transmitting end and the receiving end is a significant factor. The power budget can be calculated, for example, by the loss (10 dB) by the two AWGs, the loss
(5.5dB) by SMF (in case of 20Km), the losses (0.5dB) by any additional adapter (for example, elements being used for connecting patch cords which are used for the connection between optical elements), and the slicing loss by the two AWGs. When considering reception sensitivity (-34dBm) of the receiving end, the possible slicing loss becomes approximately 14dB. Thus, when referring to the upper graph indicating the slicing loss illustrated in Fig. 4, it may be easily understandable a wavelength value of the maximum gain of the laser diode has to be controlled to be within a detuning range of -2.0nm ~ +1.0nm.
[33] In addition, when using an AWG, as a multiplexer/de-multiplexer, having a bandwidth narrower than a bandwidth of the AWG explained in an embodiment illustrated in Fig. 4 (for example, a flat-top type AWG of 100GHz has 0.60nm as its bandwidth, while a Gaussian type AWG of 100GHz has 0.45nm as its bandwidth), the slicing loss increases further. In this case, the slicing loss may be compensated by using an optical amplifier, such as an erbium-doped fiber amplifier (EDFA) which can be used for an amplifier in a general optical telecommunications, at a front end of the receiving ends (i.e., at a position between a PD and a WDM at the receiving ends of the OLT and the ONT). Further, in case of transmitting a high-speed data of 1.25Gb/s, it is possible to transmit at a high-speed if the slicing loss described above is compensated by using an optical amplifier.
[34] In the meanwhile, a method of locking a wavelength value having the maximum gain of a multi-mode laser diode having two contacts, like an embodiment of the present invention illustrated in Fig. 4 has been disclosed in Korean Patent No.0680918, which was filed on January 27, 2005 as Korean Patent Application No. 10-2005-0007643 with a title of "The wavelength control device using Fabry-Perot laser diode with three over contact" by Chang-Hee, LEE et al. and registered on February 2, 2007. At least three or more contacts disclosed in Korean Patent No.0680918 include one ground contact and two or more injection contacts.
[35] Fig. 5 illustrates a view of a BER (Bit Error Rate) characteristic depending on temperatures in a WDM-PON, in case of using a multi-mode laser diode having two contacts according to an embodiment of the present invention as an optical transmitter. More specifically, Fig. 5 illustrates a view of a BER, in case of using a multi-mode laser diode having two contacts according to an embodiment of the present invention as a light source of an optical transmitting end of a WDM-PON, and varying injection current to be injected into the two contacts so as to lock the wavelength of the multi- mode laser diode to the center wavelength of the AWG as suggested in the present invention.
[36] A data transmission speed of 155Mbps is used and the BER is measured at an interval of 5 0 C within a temperature range between 2O 0 C and 4O 0 C, in an embodiment
of the present invention illustrated in Fig. 5. In this case, it is possible to obtain a BER value of 10 (-10 as a log value illustrated in Fig. 5), which is a target value to be accomplished when transmitting a data at a data transmission speed of 155Mb/s, within the measured temperature range. Accordingly, it may be understandable that an error- free transmission characteristic is obtained within a measured temperature range, when referring to Fig. 5. Industrial Applicability
[37] In a WDM-PON according to the present invention, it is possible to embody a
WDM-PON at a low cost by varying injected electric current of a multi-mode laser diode having a multi-contact without injecting an incoherent light source from outside.
[38] In addition, a WDM-PON according to the present invention may accomplish a highspeed transmission with lGb/s or more when the WDM-PON has enough optical power from an optical transmitter to an optical receiver, by using a transmission format having high-spectrum efficiency.
[39] Further, a WDM-PON according to the present invention may be applicable to a general optical telecommunications system.
[40] As various modifications could be made in the constructions and method herein described and illustrated without departing from the scope of the present invention, it is intended that all matters contained in the foregoing description or shown in the accompanying drawings shall be interpreted as illustrative rather than limiting. Thus, the breadth and scope of the present invention should not be limited by any of the above- described exemplary embodiments, but should be defined only in accordance with the following claims appended hereto and their equivalents.