Distribution and reception of wavelength reference in optical multichannel network

Abstract: The invention relates to a procedure and devices for distribution and reception of wavelength reference in optical multichannel networks. A wavelength reference unit transmits a wavelength reference signal to at least one node which has transmitter laser with need for a wavelength reference. According to the invention the wavelength reference signal includes a carrier, the wavelength of which is varied continuously within a wavelength band and the carrier is modulated with a signal making a clear-cut relation between the momentary wavelength of the carrier and the modulating signal. The node compares the wavelength of its transmitter laser with the carrier and with a correct wavelength to which the node shall be stabilized, at which a relation is obtained between the wavelength of the transmitter laser and the correct wavelength. The transmitter laser is controlled depending on the obtained relation (Figure 1).

Description: TECHNICAL FIELD The present invention relates to a procedure and device to distribute and receive wavelength reference in optical multichannel networks. The invention allows a stabilization of the wavelength, which is transmitted by the nodes in the network by the transmitted wavelenght being related to one in the network distributed reference wavelength, which includes a superimposed modulation signal. A measure of frequency deviation, if any, can be obtained, and the frequency of the concerned transmission laser in the node can be adjusted to wanted value. Optical telecommunications network will to an increasing extent utilize more and more wavelength channels per fibre. This so called multichannel technology can be used to increase the transmission capacity in an optical link, but also to in a more flexible way distribute and control the signal flow through the network. In local networks different wavelenght channels can be used to distribute different kinds of services or to direct information to different subscribers. PRIOR ART To make a multichannel network well operating is required that the wavelength of the signal which is transmitted on the network is stable and well separated from the wavelengths of other transmitters. Because the qualities of the light producing laser can change as time goes on due to effects of age, deviations of temperature and current, the originally correctly adjust wavelength will be influenced. Within the field a number of documents are known which describe the solution of the problem with the above mentioned effects of aging at distribution of light signals in an optical multichannel system. US 4 715 028 describes a method to distribute an optical reference signal in an optical communications network. The reference signal consists of a carrier the wavelength of which varies continuously between two known values. The carrier is introduced in the system together with a synchronization signal, which for instance, can indicate a starting point for the frequency sweep. The nodes in the network can detect possible frequency deviations by detecting the time between the synchronization pulse and the point of time when the reference signal coincides with the own frequency. This time difference can after that be utilized for adjusting the own frequency. US 4 835 782 shows a method to stabilize the frequency at a number of laser units in an optical communications system. The method is used for increasing the transmission capacity in a frequency multiplexed system. The method has as its aim to distribute a reference signal, which contains a number of synchronization pulses, which are superimposed on the reference signal on points of time, which represent those in the laser units wanted frequencies. By detecting the time difference between a synchronization pulse and the point of time when the reference signal coincides with the frequency of the laser unit, a measure of the frequency error can be obtained. By that the frequency of the laser unit can be adjusted. US 5 239 400 shows a technology for accurate generation of carrier frequency in an OFDM-system. The reference signal consists of a number of sequences with successively increasing frequency according to the resonance points for a Fabry-Perot-filter. SU 457 043 shows a device for generation of a reference frequency intended to be used in a spectrum analyzer. The device generates a reference spectrum with a number of superimposed signals which represent a number of frequency components. GB 2260 047 shows a frequency marking method for producing a go frequency coincidence with a monitored frequency. This is done by letting a reference frequency be scanned across a scanning band. During each scanning half-cycle it affects a go path and a return path according to a known law to produce a go frequency coincidence. A marker interval is measured which is the time elapsed between the two coincidences. This interval marks the monitored frequency and enables the frequency to be locked by comparison with a set point interval. A disadvantage with the known technology is that it requires extra synchronization pulses, which are transmitted separately from the wavelength reference light. Besides, linearity of the wavelength sweep is required. In a growing network there may also be a need to increase the wavelength interval between the start - and stop wavelength, which implies that the system reference source and terminals in the network also must be changed. The present invention solves these problems by using a superimposed modulation signal on the sweeping wavelength reference light. By that a well-defined predetermined relation between the modulation signal in the reference wavelength signal and the wavelength of the reference wavelength signal is established. This means that no extra synchronization signal need to be transmitted, and that an absolute wavelength control can be achieved without the need of using the start- and stop wavelengths of the reference source in the nodes. SUMMARY OF THE INVENTION The present invention accordingly provides a procedure to distribute and receive wavelength reference in optical multichannel networks comprising a wavelength reference unit which transmits a wavelength reference signal to at least one node which has a transmitter laser with need for a wavelength reference. According to the invention the wavelength reference signal comprises a carrier, the wavelength of which is continuously varied within a wavelength band and the carrier is modulated with a signal so a well-defined predetermined relation between the instantaneous wavelength of the carrier and the modulated signal is achieved. The node compares the wavelength of its transmitter laser with the carrier and with a predetermined wavelength to which the node shall be stabilized, at which a relation between the wavelength of the transmitter laser and the predetermined wavelength is obtained. The transmitter laser is controlled depending on the obtained information. The invention also provides devices for distribution and reception of wavelength reference in optical multichannel network. The invention is presented in details in enclosed patent claims. BRIEF DESCRIPTION OF THE FIGURES The invention is in the following described in details with references to enclosed figures in which Figure 1 is a block diagram over an embodiment of a node according to the present invention, and Figure 2 is a diagram over the wavelength variation in the carrier. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS To get optical multichannel networks to function it is required that the wavelength of the signal which an optical transmitter transmits is stable and well separated from the wavelengths of other transmitters. In other case there is a risk that a transmitter drifts into the band of wavelengths of another transmitter or causes strong disturbances on the channels. At the same time there is another aim to arrange the channels as closely as possible in order to increase the degree of exploitation of the totally available band of wavelengths. The light source in an optical transmitter for a multichannel system consists of a diode laser the wavelength of which of the transmitted light depends on among other things the driving current of the laser, temperature and age. Driving current and temperature can be adjusted to correct values at the installation of the transmitter, but since the qualities of the laser change with time, the wavelength will also change with time. In addition aging effects in temperature sensors and current stabilization circuits can cause the wavelength to drift. The optimal wavelength separation between the channels depends on several factors such as data rate, link distance, type of fibre, optical effect level, type of optical receiver etc. In broad outline one can say that the wavelength stability, which will be required from different systems will be in the interval 0,01-0,2 nm. This can be compared with the wavelength drift of a typical laser, which is about 0,15 nm/°C and about 0,01 nm/mA. In order to cope with the strongest wavelength stability is accordingly required a temperature stability better than 0,1°C and a current stability better than 1 mA. Consequently there is a need for control equipment, which can adjust the drift in the wavelength of the transmitter laser in spite of effects of aging and drift of temperature and current. In an optical network there can be a lot of transmitters, which transmits at a number of different wavelengths. In principle it is possible to equip every transmitter with an equipment for wavelength stabilization which contains its own wavelength reference. In practice this would be an expensive solution. A cheaper way is in some way to distribute the wavelength reference via the network itself, for instance in the form of one or more reference wavelengths which the local equipment can utilize to stabilize its wavelength. By that a lot of transmitters can use the same wavelength reference unit. All the nodes in the network where there is a need to stabilize the wavelength of an optical transmitter have contact with a wavelength reference unit via an optical fibre. The practical design of the wavelength stabilization equipment in the node depends to some extent on whether this fibre consists of the same fibre as the transmitter uses for transmitting its carrier, or consists of a separate optical fibre separated from the single network itself. In the present description is supposed that the wavelength reference is distributed via a separate optical fibre. The output signal from the wavelength reference unit consists of an optical carrier the wavelength of which varies continuously with time between a minimum and a maximum value. The difference between the minimum and the maximum value is called the wavelength band of the unit and all transmitters, which shall be stabilized, must have a wavelength within this band. Exactly how the wavelength varies with time is not critical, but from a practical point of view, a saw tooth shaped variation can be suitable. An example of wavelength variation as function of time is shown in Figure 2. In addition to the variation in wave length, the carrier from the wavelength reference unit is modulated in such a way that there is a well-defined predetermined relation between the wavelength of the carrier and the modulating signal. The aim with this is that the nodes in the network shall be able to read off the wavelength of the reference signal by reading the modulation signal. A number of different modulation methods are possible. One variant is intensity modulation with a sine shaped signal the frequency of which varies with time in a way that the frequency of the intensity modulation in each moment is directly related to the wavelength of the carrier. Another variant is pulse width modulation of the carrier of the wavelength reference unit, where the pulse width is directly related to the wavelength. In the preferred embodiment of the invention intensity modulation is used, as is explained below. In the nodes are registered the moments when the wavelength of the carrier from the wavelength reference unit coincides on one hand with the wavelength of the transmitter laser, and on the other with the wavelength to which the transmitter laser shall be stabilized. The time difference between these two occurrences constitutes a measure of the wavelength error of the transmitter laser and is used by the wavelength stabilization equipment in the node for correction of the wavelength of the transmitter. The transmitter laser has a predetermined wavelength when the time difference is zero. In Figure 1 is shown an embodiment of wavelength stabilization equipment in the node. The wavelength reference is distributed via a special fibre and the carrier from the wavelength reference unit is modulated with a sine-shaped intensity modulation, the frequency of which is a function of the wavelength of the carrier (Figure 2). The mathematical relation between wavelength and modulation frequency (or pulse width) can be formulated in different ways. A proportional relation as for instance frequency [Hz] = wavelength [nm] x K where K is a suitable constant, is simple but gives in some cases not sufficiently accurate wavelength adjustment. By instead selecting a relation of the type frequency [Hz] = (wavelength [nm] - K1) x K2 where K1 and K2 are suitably selected constants, the precision in the wavelength adjustment can be improved to wanted level. The wavelength reference signal, which is coming in to the node is divided by means of an opto coupler OK1 which distributes 50% each of the signal to optical input stages to the node. The output signal from the transmitter laser is also divided into two parts by an opto coupler OK2, where the strongest part, for instance 90% is transmitted out on the network, whereas the weaker part is passed on to the opto coupler OK1, where it is also divided into two equal parts and is mixed with the wavelength reference signal on the photodiodes at the input stages. Due to the fact that the light of the wavelength reference and the light of the transmitter laser interfere with each other, the electrical signal from the optical input stages will contain a signal SM which is proportional to the modulation of the wavelength reference and a signal SH, the frequency of which is equal to the frequency difference between the light of the wavelength reference and the light of the transmitter laser according to the heterodyne principle. Owing to that the SM-signal from the two input stages has the same phase whereas the SH-signal has opposite phase, the SM- and SH-signals are separated from each other in a following addition- and subtraction stage. The SH-signal after that passes a lowpassfilter LP1, which only allows a signal to pass when the SH-signal has a low frequency. Since the wavelength of the wavelength reference varies with time the level of the output signal after LP1 will also vary with time. Maximum occurs every time the wavelength reference and the transmitter laser has the same wavelength. A following top detector TD1 generates a short pulse PH exactly when the SH-signal reaches this maximum. The pulse PH passes on to a logical unit, the function of which is described in the following. The signal SM first goes to a multiplier, where it is multiplied by a sine signal, the frequency FM of which corresponds to the wavelength to which the transmitter laser shall be stabilized. In the multiplier a signal is generated the frequency of which is equal to the frequency difference between FM and the modulation frequency of the wavelength reference. The low pass filter LP2 only allows a signal to pass when this frequency difference is small. Since the wavelength and by that the modulation frequency of the wavelength reference varies with time, the level of the output signal after LOP2 will also vary with time and maximum occurs each time the modulation frequency and FM are equal. In a following top detector TD2 a short pulse PM is generated exactly when the frequency difference is zero. The pulse PM after that is passed on to the logic unit. The invention utilizes the fact that the pulse PH occurs in time when the wavelength reference and the transmitter laser have the same wavelength, whereas the pulse PM occurs when the wavelength reference has the wavelength to which the transmitter laser shall be stabilized. If the pulses PH and PM occur at the same time, the transmitter laser is correct in wavelength. In other case the transmitter laser is under or over in wavelength depending on whether PH comes before or after PM. The task of the logical unit is to adjust the wavelength of the transmitter laser to make PH and PM simultaneous. This is performed by controlling the power feeding or temperature of the transmitter laser. Exactly how this is performed depends on the construction of the transmitter laser, as is known to persons with expert knowledge in the field. The logical unit must know which wavelength the transmitter laser shall have. This information the logical unit will receive from the main control system of the network. Starting out from wavelength data the logical unit calculates the frequency FM and generates a sine signal with this frequency to be fed to the multiplier.

Claims: Procedure to distribute and receive wavelength reference in optical multichannel network including a wavelength reference unit which transmits a wavelength reference signal to at least one node which has a transmitter laser with need for a wavelength reference, wherein the wavelength reference signal includes a carrier, the wavelength of which is varied continuously within a wavelength band, the node compares the wavelength of its transmitter laser with the carrier and with a predetermined wavelength, to which the node shall be stabilised, a relation being obtained between the wavelength of the transmitter laser and the predetermined wavelength, and the transmitter laser is controlled depending on the obtained relation, characterized in that the carrier is modulated with a signal making a well-defined predetermined relation between the instantaneous wavelength of the carrier and the modulating signal. Procedure according to claim 1, characterized in that a part of the output signal of the transmitter laser is mixed with the wavelength reference signal in order to obtain a first signal (SH) the frequency of which is equal to the frequency difference between the the signal of the transmitter laser and the wavelength reference signal, and a second signal (SM) which is proportional to the modulation of the wavelength reference signal, that the point of time when the first signal (SH) has a maximum is detected, that the second signal (SM) is multiplied with a signal the frequency (FM) of which corresponds to the predetermined wavelength, in order to produce a third signal the frequency of which is equal to the difference between the frequency (FM) which corresponds to the predetermined wavelength and the modulation frequency of the wavelength reference signal, that the point of time when the third signal has its maximum is detected and that the transmitter laser is controlled making the mentioned points of time to coincide. Procedure according to claim 2, characterized in that information about predetermined wavelength for calculation of corresponding frequency is provided by the control system of the network. Procedure according to any of the previous claims, characterized in that the carrier is modulated by intensity modulation with a sine shaped signal the frequency of which varies with time in such a way that the frequency of the modulation is directly related to the wavelength of the carrier. Procedure according to claim 4, characterized in that the frequency f of the intensity modulation is a function f of the wavelength λ of the carrier, where the function f is f = λ x K or f = (λ-K1) x K2 where K, K1 and K2 are constants. Procedure according to any of the claims 1-3, characterized in that the carrier is modulated by pulse width modulation in such a way that the pulse width is directly related to the wavelength of the carrier. Procedure according to any of the previous claims, characterized in that the wavelength of the carrier varies as a sawtooth shaped function of time within the wavelength band. Device for distribution of wavelength reference in optical multichannel network including a wavelength reference unit for transmission of a wavelength reference signal to at least one node which has a transmitter laser with need for a wavelength reference, which wavelength reference unit has a device to produce the wavelength reference signal with a carrier, the wavelength of which varies continuously within a wavelength band, characterized by a device to modulate the carrier with a signal making a well-defined predetermined relation between the instantaneous wavelength of the carrier and the modulating signal. Device for distribution of wavelength reference according to claim 8, characterized by a device to provide information about the predetermined wavelength of respective transmitter laser for calculation of corresponding frequency in the node. Device for distribution of wavelength reference according to claims 8 or 9, characterized in that the modulation device operates by intensity modulation with a sine shaped signal the frequency of which varies with time making the frequency of the modulation directly related to the wavelength of the carrier. Device for distribution of wavelength reference according to claims 8 or 9, characterized in that the modulation device operates by pulse width modulation in such a way that the pulse width is directly related to the wavelength of the carrier. Device for reception of wavelength reference in optical multichannel network including a wavelength reference unit for transmission of wavelength reference signal to at least one node, which has a transmitter laser with need for a wavelength reference, a device for comparison of the wavelength of the transmitter laser with the wavelength of a modulated wavelength in the wavelength reference signal and with a predetermined wavelength, to which the node shall be stabilised to obtain a relation between the wavelength of the transmitter laser and the predetermined wavelength, and a device to control the transmitter laser depending on the obtained relation, characterized in that it comprises devices (OK1, OK2) to mix a part of the output signal of the transmitter laser with the wavelength reference signal to obtain a first signal (SH), the frequency of which is equal to the frequency difference between the signal of the transmitter laser and the wavelength reference signal, and a second signal (SM), which is proportional to the modulation of the wavelength reference signal, a device (TD1) for detection of the point of time when the first signal (SH) has a maximum, a device for multiplication of the second signal (SM) with a signal, the frequency (FM) of which corresponds to the predetermined wavelength, to produce a third signal, the frequency of which is equal to the difference between the frequency (FM) which corresponds to the predetermined wavelength and the modulation frequency of the wavelength reference signal, and a device (TD2) for detection of the point of time when the third signal has its maximum, said control device being arranged to control the transmitter laser in such a way that the mentioned points of time coincide.

Inventors: OESTLUND LEIF, OESTLUND, LEIF, Östlund, Leif
Applicants: TELIA AB
Application year: 1996
Main classification: H04B10
IPC codes: H04B10