WO2003081812A1

WO2003081812A1 - Stabilizing optical sources in a communication system___________

Info

Publication number: WO2003081812A1
Application number: PCT/IL2003/000210
Authority: WO
Inventors: Alon Ashkenazi
Original assignee: Axonlink (Bvi) Corporation; Axonlink (Israel) Ltd.
Priority date: 2002-03-21
Filing date: 2003-03-13
Publication date: 2003-10-02
Also published as: IL148810A0; AU2003216702A1

Abstract

An optical device operative to stabilize an output of at least one optical source at a predefined wavelength. The devices comprises: at least one optical source, a calibrated reference source, an optical heterodyne receiver which is operative to detect a wavelength difference between the reference source and each of the at least one optical source, and means for adjusting the wavelength of each of the at least one optical source according to the wavelength difference.

Description

STABILIZING OPTICAL SOURCES IN A COMMUNICATION SYSTEM

Field of the Invention

The present invention relates to a stabilization device for optical sources having a certain frequency relation between them. A typical application for such an optical stabilizer is in telecommunication networks.

Background of the Invention Optical transmissions sources based on lasers have long been known in the art. These optical sources are usually arranged to operate at certain wavelengths . In many telecommunication systems a multiplicity of optical sources are used to transmit plurality of signals over a single optical fiber. Such systems are generally known as WDM - Wave Division Multiplexing systems. In WDM systems, optical sources of each optical channel are typically controlled to operate at specific wavelengths that typically are equally spaced over a specifiσ wavelength range (see for example ITU Recommendation G.692) . In DWDM - Dense Wave Division Multiplexing systems, the spacing between two adjacent optical channels is typically 100 GHz wherein some new systems use even a denser spacing of 50 GHz. It is important to accurately monitor and control the frequency of each optical channel so that the transmission that is carried along that channel will not interfere with that of adjacent channels and will be properly received by a corresponding optical receiver at the other end of an optical link. Another problem well known in the art that is associated with such systems, is, that the frequency of a laser based optical source is affected by the environmental temperature and by the aging of the laser. Therefore it is necessary to continuously monitor and maintain the frequency of each optical source throughout its operation. In new optical communication systems wherein the spacing between the optical chanrels becomes ever smaller, these problems are intensified. Spacing of 50GHz is already standardized in ITU Recommendation G.692. Denser spacing of 25GHz is also being implemented on an experimental basis. See for example, Y. Yamada et al . "25 GHz spacing ultra-dense WDM transmission experiment of 1 Tbit/s (100 WDMxlO Gbit/s) over 7300 km using non pre-chirped RZ format" IEE Electronics Letters, 1999, vol.35, no.25, p 2212-2213. Even αenser spacing of 12.5GHz is being considered. Such systems require fine stabilization of optical sources. Spacing of 12.5GHz corresponds to wavelength spacing of about 0.1 nm. Optical sources that operate in suc. a communication system need to be stabilized to prevent crosstalk between adjacent optical channels. Therefore tne most important parameter is the accuracy of the wavelength distance between two adjacent channels. The accuracy of the wavelength of each optical source should be better by at least one order of magnitude than the wavelength distance between two adjacent optical sources. Therefore in a communication system that uses spacing of 12.5GHz - .e. O.lnm, the stabilization accuracy of each optical source should be at least O.Olnm and in most cases it would be desirable to achieve accuracy of O.OOlnm. One example of a communication system that is using 12.5GHz spacing is described in Israeli patent Application No. 145,859. The system described therein is optimized for access networks where the cost is a very important factor and the bit rate that is carried by each optical channel is relatively low (1.25Gbιt/s or lower).

A number of methods have been described in the literature to monitor and control the frequency of lasers. The most common method in the art is to use a pair of Etalon filters in each laser source. The major disadvantages of this method are the additional cost of these Etalon filters and their fixed nature. Therefore this method enables the stabilization of a laser source at a predetermined wavelength and coes not offer an option for flexible tunability. Another stabilization method is described in US 5,299,212 and is based on in line grating of an optical fiber. Again, this method is complex and controls the laser source so that its frequency is maintained at a predetermined value defined by the optical fiber grating. Yet another method for controlling the wavelength of a laser based optical source is disclosed in U.S. Patent No. 6,233,262. In this method the wavelength of the laser is monitored by two optical filters that are connected to the output of the laser. These two optical filters have each a different frequency response. The output of the two optical filters is measured and the ratio between tie outputs of the optical filters is calculated. The system is calibrated so that each desired wavelength corresponds to a specific power ratio between the outputs of the optical filters . Therefore the laser source can be controlled to produce any desired wavelength according to the ratio of the outputs of the optical filters. This method is designed to operate over a wide range of optical wavelength (e.g. of about 40 nm) . The power ratio of the outputs of the two filters is substantially non-linear over the wavelength range, thus providing different accuracy performance at each wavelength. Another method for controlling the wavelength of an optical source is disclosed by US 5,861,975. In this method the distance between two optical sources is controlled by multiplexing the two optical sources and optical heterodyne detection of the multiplexed optical signal. The output of the optical heterodyne detection provides the frequency difference signal between the two optical sources. While one of the optical sources is considered as reference, the wavelength/frequency of the other optical source is controlled so that the wavelength/frequency difference between the two sources remains at a prescribed value. However, one of the major drawbacks of this method is the inherent need for an O/E (optical/electrical) converter that should be able to respond to the high frequencies of tenth or hundreds of GHz that are the typical spacing between optical sources in WDM systems. Such converters are very complex and expensive. Additionally, US 5,861,975 does not disclose how this method can be implemented to control the spacing between more than two optical sources.

It is therefore required to provide a device and method that overcome these prior art problems and are operative in positioning and stabilizing, in a cost-effective manner, the wavelength of optical sources that are densely spaced.

Summary of the Invention

It is therefore an object of the present invention to provide a device and a method operative to substantially reduce any crosstalk between adjacent channels of an optical communication system having densely spaced channels. It is another object of the present invention to provide a device and a method operative to provide a different and/or variable spacing between optical channels of the same communication system. Further objects and features of ne invention will oecome apparent from the following description and the accompanying drawings .

In accordance with the present invention there is provided an optical device operative to stabilize an output of at least one optical source at a predefined wavelength, comprising: at least one optical source; a calibrated reference source; an optical heterodyne receiver whic" is operative to detect a wavelength difference bet, een said reference source and each of said at least c^~e optical source; and means for adjusting the wavelength of each of said at least one optical source according to said wavelength difference.

The term "reference source" as used herein, is used to denote an optical reference source. Similarly, the term "reference signal" as used herein is used to denote a"> optical reference signal. Preferably, this reference source is a source that is pre-calibrated over a predefined wavelength range.

The present invention also encompasses a mode of operation wherein the calibrated reference source is not co-located with the optical source that is being stabilized and/or with the optical heterodyne receiver that is used to detect the wavelength difference . When an optical source is used for transmission of optical signals over an optical network, it propagates optical signals in the network and their wavelength can be compared to the wavelength of the reference source at any required location in the network.

According to a preferred embodiment, the optical device of the invention comprises a plurality of optical sources, and the output of each of the plurality of optical sources is stabilized at a different wavelength.

According to yet another preferred embodiment, the adjusting means are operative to adjust the wavelength of tne optical sources in such a way tr.at the wavelengtn difference is substantially smaller tnan the wavelength distance between any two adjacent optical sources. The present invention also encompasses controlling the adjusting means either by a local control located at the location where the adjustment is affected, and in addition or in the alternative, such a controller may be located at a remote location, and the appropriate control commands be transmitted to the site wnere the adjustment s affected.

By another embodiment of the invention, the output of each of the optical sources is equally spaced on the \avelength scale. Alternatively, the outputs of the optical sources are spaced so that tne space between a first pair of adjacent optical sources is not equal to the space between a second pair of adjacent optical sources .

In accordance with another aspect of the invention, a method for stabilization of an output of at least one optical source at a predefined wavelength is provided.

The method comprising: using an optical heterodyne receiver to detect a wavelength difference between a reference signal and a signal received from each of said at least one optical source; and adjusting the wavelength of said at least one optical source according to said wavelength dif erence .

By a preferred embodiment of the invention, the stabilization method is carried out for a plurality of optical sources .

In accordance with still another embodiment of the invention, the predefined wavelength is different for the output of each of the optical sources. By yet another preferred embodiment, the method is directed to ensure that the wavelength difference is substantially smaller than the wavelength distance between any two adjacent optical sources out of the plurality of optical sources. Preferably, the adjusting step is designated to reduce the wavelength difference to a value that is less than about O.Olnm.

Preferably, by the method provided the output of each of the plurality of optical sources is located at any predefined location on the wavelength scale. In the alternative, the output of the plurality of optical sources is equally spaced on the wavelength scale.

In accordance with still another; preferred embodiment, the method provided by the present invention further comprises the step of: periodically monitoring and stabilizing the wavelength of any active optical source out of said at least one optical source.

Brief Description of the Drawings ?ig. 1 illustrates schematically the relevant parts of an optical device constructed in accordance with the present invention that is capable of stabilizing the wavelength of the output of each of a plurality of optical sources; Fig. 2 shows in a flow-chart form the procedure that is invoked during the initialization phase of the optical device in accordance with the present invention; Fig. 3 shows in a flow-chart form the stabilization procedure that is periodically performed during the normal operation of the optical device in accordance with the present invention;

Detailed Description of the Invention

The present invention relates to an optical device and method for positioning and stabilizing the wavelength of an optical source and/or a plurality of optical sources. The present invention is particularly effective m positioning and stabilizing a plurality of optical sources, with very high accuracy, wherein it is required to maintain a specific, relative relation between the operating frequencies of optical sources . The invention will now be demonstrated in the following non-limiting example.

Fig. 1 shows a simplified illustration of an optical device comprising sixteen optical sources 2-1 through 2-16, implemented by semiconductor lasers, operative in a WDM communication system. The opt_cal device also comprises a seventeenth optical source 4 used as a reference source, and according to this example is also implemented by a semiconductor laser. Reference source 4 may be implemented by using the same type of semiconductor laser as used for optical sources 2-1 through 2-16. The wavelength of optical sources 2-1 through 2-16 as well as the wavelength of reference source 4 is controlled by varying the temperature of each semiconductor laser or by any other applicable methods known in the art per se . In commercially available semiconductor lasers such as Mitsubishi FU-68PDF-V510M79B the wavelength can modified by changing the temperature via a built-in TEC - a thermoelectric cooler, or by any other means for adjusting the wavelength as known m the art per se. In this particular example, the wavelength is adjusted within about 200GHz, «nici corresponds to 16 optical channels with uniform spacing of 12.5GHz . The temperature of the laser and thus its wavelength, are typically derived from a measurement carried by an internal thermistor. The value of tne thermistor is derived by measuring the voltage on it.

The output of each of the optical sources 2-1 through 2-16 is split by optical splitters 6-1 through 6-16, such as a 90/10 coupler, nich passes 90% of the output power of each of the optical sources 2-1 through 2-16 onward for modulation and transmission while 10% of the output power is coupled to tie input of an optical coupler 8. The output of the reference source 4 is also connected to the input of optical coupler 8. Therefore the output of coupler 8 comprises a sum of sample signals of optical sources 2-1 through 2-16 taken together with tne signal of reference source 4. Tne output of optical coupler 8 is connected to a heterodyne receiver 10 that is operative to produce electrical signals at frequencies that correspond to the frequency difference between the reference signal and each of the sixteen optical signals at its input. The electrical output of heterodyne receiver 10 is connected to filter 12. Filter 12 is operative as a band-pass filter to reject DC signals produced by heterodyne receiver 10 and to selectively detect signals that are below 100MHz. As will be described below, optical sources 2-1 through 2-16 are controlled to generate optical signals that are spaced for example 12.5GHz from each other. When reference source 4 is controlled to a wavelength that is close, within less than 100MHz, to the wavelength of one of the optical sources 2-1 through 2-16 the output of heterodyne receiver 10 will pass filter 12 and will be detected by detector 14. The DC signal at the output of detector 14 which is converted to digital form by A/D 16 is used to determine that frequency difference between reference source 4 and one of the optical sources 2-1 through 2-16 s below 100MHz. Therefore optical sources 2-1 through 2-16 are stabilized within less tnan 100MHz relative to reference source 4. Microcontroller 18 is operative to receive the output of A/D converter 16 and to control optical sources 2-1 through 2-16 via D/A converters 20-1 through 20-16. Microcontroller 18 is also operative to control reference source 4 via D/A converter 22. D/A converters 20-1 through 20-16 and D/A converter 22 are each operative to control the corresponding TEC and in turn the temperature of the corresponding semiconductor laser. In this manner, microcontroller 18 controls the wavelength of optical sources 2-1 through 2-16 and reference source 4. The temperature of each of the optical sources 2-1 through 2-16 and of reference source 4 is monitored by microcontroller 18 by examining the voltage of a thermistor located in each the optical sources 2-1 through 2-16 and in reference source 4. The voltage of a thermistor located in each the optical sources 2-1 through 2-16 and in reference source 4 is connected via analog multiplexer 24 to A/D converter 26. The microcontroller is operative to control analog multiplexer 24 to select the voltage on the thermistor to be measured. The selected thermistor voltage is converted to digital form in A/D converter 26 and transferred to microcontroller 18. The thermistor values of reference source 4 are calibrated to correspond to wavelength values of reference source 4 over the thermal control range of the semiconductor laser.

It should be appreciated that the position of the wavelength of each of the optical sources 2-1 through 2-16 can be variable and is independent of the location of the wavelength of the other optical sources. The wavelength position of each of the optical sources 2-1 through 2-16 is only dependent on the position of the reference source 4 during the stabilization of a specific optical source. Therefore according to an embodiment of tne present invention, a communication system may send control commands to microcontroller 18 to stabilize each of the optical sources 2-1 through 1-16 at any desired vavelength. It is true that a typical spacing in WDM systems is uniform, such as 12.5GHz, between adjacent optical sources. However, the device and method described m this invention enables any desired spacng in positioning the wavelength of optical sources 2-1 through 2-16. For example, it is possible to have in one system a combination of two types of spacing. For examp_e, one group of optical sources may be stabilized at a spacing of 12.5GHz while another group of optical sources may be stabilized at a spacing of 25 GHz.

The initialization procedure that is performed by microcontroller 18 at the beginning of the operation is described in Fig.2. A sequence counter j is initialized to the value of 1. As will be appreciated by those skilled in the art, the following procedure u ll be repeated /nutates mu tandi s 16 times, for each of the optical sources 2-1 through 2-16. However, for tne sake of simplicity, it will be described only once for optical source 2-1.

Optical source 2-1 is activated while optical sources 2-2 through 2-16 are deactivated and not producing any light signal. Reference source 4 is activated and positioned by microcontroller 18 at the wavelength that is equal to the desired wavelength for optical source 2-1. Microcontroller 18 varies the wavelength of optical source 2-1, preferably by small steps, until a maximum signal is produced at the output of detector 14. This confirms that optical source 2-1 is set at the desired wavelength wit.in O.OOlnm. The setting for optical source 2-1 thus obtained is stored and optical source 2-1 is deactivated.

This procedure is repeated for optical sources 2-2 through 2-16, while each time only one optical source is activated along with the reference source maintained at the corresponding wavelength, while the other optical sources are deactivated.

After completion of the initialization procedure of Fig.2, normal operation begins.

At the beginning of normal operation, each of the optical sources 2-1 through 2-16 is activated according to the requirements of the communication system. Each one of the activated optical sources is controlled to generate an optical signal at a prescribed • wavelength, according to the setting that was determined during the initialization procedure. During normal operation any of tne optical sources 2-1 through 2-16 may be activated or deactivated according to the operating requirements of the communication system.

During normal procedure a wavelength stabilization procedure is periodically (or upon a specific requirement) performed by microcontroller 18. A periodical wavelength stabilization procedure of the optical sources 2-1 through 2-16 is described in Fig.3. In order to carry out this procedure on a periodic basis, a wavelength stabilization procedure timer is restarted at the beginning of each wavelength stabilization procedure. This timer is set, for example, to 10 seconds. A wavelength stabilization sequence is repeated for each of the active optical sources. Again as described before, for the sake of simplicity we shall describe now in details the wavelength stabilization sequence for optical source 2-1 out of the full wavelength stabilization procedure. Firstly, it is confirmed that this optical source is active. Then, reference source 4 is set to the wavelength of optical source 2-1. Then scanning with reference source 4 is performed to find the exact wavelength of optical source 2-1 by detecting the maximum at the output of detector 14. If the maximum is detected at the correct wavelength position of reference source 4, then the wavelength stabilization sequence for optical source 2-1 is completed. However if a wavelength deviation is detected, a correction setting is calculated. The correction is performed gradually in order to enable proper tracking of a corresponding receiver at the other end of a communication system. The scanning and correction continues until the wavelength of optical source 2-1 is set in the correct position. As mentioned above, this sequence is repeated for each of the active optical sources 2-1 through 2-16. After completing a wavelength stabilization sequence for each of the active optical sources 2-1 through 2-16, the wavelength stabilization procedure is reactivated when the wavelength stabilization procedure timer expires.

The present invention has been described using non-limiting detailed descriptions of embodiments tnereof that are provided by way of example and are not intended to limit the scope of the invention. It should be understood that features and/or steps described with respect to one embodiment may be used with other embodiments and that not all embodiments of the invention have all of the features and/or steps shown in a particular figure or described with respect to one of the embodiments. Variations of embodiments described will occur to persons of the art.

It is noted that some of the above described embodiments describe the best mode contemplated by the inventors and therefore include structure, acts or details of structures and acts that may not be essential to the invention and which are descπoed as examples. Structure and acts described herein are replaceaole by equivalents, which perform the same function, even if the structure or acts are different, as <nown in the art. Tnerefore, the scope of the invention is limited only by the elements and limitations as used n the claims. When used in the following claims, the terms "comprise", "include", "have" and their conjugates mean "including out not limited to" .

Claims

1. An optical device operative to stabilize an output of at least one optical source at a predefined wavelength, comprising: at least one optical source; a calibrated reference source; an optical heterodyne receiver whic.n is operative to detect a wavelength difference between said reference source and each of said at least one optical source; and means for adjusting the wavelength of each of said at least one optical source according to said wavelength difference .

2. An optical device according to cla_m 1 wherein said reference source is pre-calibrated over a predefined wavelength range.

3. An optical device according to claim 1 and 2 comprising a plurality of optical sources, wherein the output of each of said plurality of optical sources is stabilized at a different wavelength.

4. An optical device according to claim 3 wherein said adjusting means are operative to adjust the wavelength of said plurality of optical sources so that said wavelength difference is substantιal_y smaller than the wavelength distance between any two adjacent optical sources .

5. An optical device according to claim 4 wherein said output of each of the plurality of optical sources is equally spaced on the wavelength scale.

6. An optical device according to claim 4 wherein said optical sources are spaced so that tne space between a first pair of adjacent optical sources is not equal to the space between a second pair of adjacent optical sources .

7. A method for stabilization of an output of at least one optical source at a predefined wavelength comprising: using an optical heterodyne receiver to detect a wavelength difference between a reference signal and a signal received from each of said at least one optical source; and adjusting the wavelength of said at least ore optical source according to said wavelength difference.

8. A method according to claim 7 wherein the stabilization is carried out for a plurality of optical sources .

9. A method according to claim 3 wherein said predefined wavelength is different for said output of each of said plurality of optical screes.

10. A method according to claim 9 wherein said wavelength difference is substantially smaller than the wavelength distance between any two adjacent optical sources out of said plurality of optical sources.

11. A method according to claim 10 wherein the adjusting step is designated to reduce said wavelength difference to a value that is less than about O.Olnm.

12. A method according to claim 10 wherein the output of said plurality of optical sources is equally spaced on the wavelength scale.

13. A method according to claim 10 wherein the output of each of said plurality of optical sources is located at any predefined location on the waveleⁿgth scale.

14. A method according to any one of claims 7 - 13 further comprising the step of: periodically monitoring and stabilizing the wavelength of any active optical source out of said at least one optical source.

15. An optical device according to claim 1, substantially as described herein with reference to the drawings .

16. A method according to claim 7, substantially as described and exemplified herein.