Optical signal power detection with signature bit pattern in WDM systems

Abstract: The power of an optical signal (s 1 ) travelling on a channel (λ 1 ) of a WDM transmission system, is measured using a signature bit pattern (s BP1 ) which is inserted in the frame of the optical signal (s 1 ). The power level of s BP1 is adjusted at the launching point to a predetermined ratio (m) with the power of the optical signal. At a point of interest, the fiber is tapped and a fraction of the tapped signal, that includes a corresponding fraction of s BP1 , is converted to an electrical signal. The fraction of s BP1 is extracted from a the electrical signal and power of s BP1 is measured. This gives the optical power of s 1 as (m) is known and also the calibration constant for the respective channel (λ 1 ) is known. The method can be applied for any and all channels of the WDM transmission system.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention is directed to a method for monitoring the performance of optical WDM systems and in particular to an optical signal power detection method using a signature bit pattern. Background Art In optical transmission systems, various parameters such as power are measured to obtain information on the operating conditions of the transmission link. The fiber optic cables are tested by measuring the power losses associated with the light transmission, and thus, determine potential transmission errors. In a WDM system, because different wavelengths of light have different attenuation characteristics, it is important to determine the attenuation of the fiber optic cable with respect to a specific wavelength of light used for a particular transmission channel. It is important to detect accurately the optical power of individual optical signals for many reasons, such as improved control of optical amplifiers, signal tracking at the optical layer, monitoring the accumulation of optical noise in a link with cascaded amplifiers, etc. It is known to monitor the input and output of an optical amplifier in order to control the gain. To this end, fractions of the input and output signals are coupled out by taps (couplers) and detected by photodiodes. The electrical signals, recovered after this detection, are then used by the power monitor as needed. Since the power of the coupled out signal is very low in the case of digital systems, the power monitors required to detect and process this low signal are rather complex. To date, the only other method for detecting the optical power of signals without using expensive optical filters is to amplitude modulate the optical signal to a controlled modulation depth with a signal (dither) unique to the respective transmission system. This method, disclosed in United States Patent No. 5, 513,029 by Kim Roberts, issued on April 30, 1996 and assigned to Northern Telecom Limited, requires additional optical components, such as an external modulator and optical attenuators, and also requires electronics and real time control software at the transmitter, to both apply the amplitude modulation and to detect it, in order to accurately control the modulation depth. SUMMARY OF THE INVENTION It is an object of the present invention to provide a method for detecting the optical power of a digital optical signal in the presence of other optical signals at different wavelengths, without using expensive fixed or tracking optical filters. Accordingly, there is provided a method for measuring the power of an optical signal (s 1 ) travelling on a first channel (λ 1 ) of a WDM transmission system, comprising the steps of, generating a signature bit pattern (s BP1 ), adjusting the power level of the signature bit pattern to a predetermined ratio (m) with the power of the optical signal, inserting the signature bit pattern (s BP1 ) into the frame of the optical signal (s 1 ) and transmitting same along a span of transmission medium, measuring the power of the signature bit pattern (s BP1 ) at a point of interest on the span, and determining the optical power of the optical signal (s 1 ) in the point of interest. The advantage of this invention is that it provides a simple method for determining the optical power in a WDM system, whereby no additional electronics, expensive optical components and real time software are required at the transmitter side, resulting in considerable savings in circuit pack layout space, cost and development time. BRIEF DESCRIPTION OF THE DRAWINGS The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of the preferred embodiment, as illustrated in the appended Figure 1 which shows a block diagram of a WDM system according to the invention. DESCRIPTION OF THE PREFERRED EMBODIMENT For this invention, a unique signature bit pattern is inserted in a digital optical signal, where the rms (root mean square) of the signature bit pattern has inherently a fixed ratio to the optical power. Detecting the rms of this signature bit pattern means the optical power of the optical signal can be determined. As well, the optical power of the optical signals co-propagating along the same fiber span may be detected at the same time, when each optical signal is given a unique signature pattern. The signature bit pattern for each co-propagating signal is designed to be detected independently from each other. Figure 1 shows a block diagram of a WDM system using the signature bit patterns according to the invention. The WDM system of Figure 1 shows a plurality of optical signals s 1 - s n propagating in the same direction. It is to be understood that the invention applies also to bidirectional optical WDM systems, such as for example to SONET/SDH 8-wavelength bidirectional systems. Signals s 1 - s n are multiplexed in an optical multiplexer 12 to obtain a multichannel signal s on fiber 11. A signature bit pattern, shown at s BP1 , is inserted into the frame of optical signal s 1 . If we note the average optical power of s 1 with 'P 1 ', the average optical power of s BP1 with 'P BP1 '. and the ratio between the powers of s BP1 and s 1 with 'm', we have the following relationship: m = P BP1 / P 1 or, P BP1 = m x P 1 As the signature bit pattern is inserted in the frame of the digital optical signal, ratio 'm' remains constant along the entire path. This is however true only if either the extinction ratio for the optical signal remains constant and known, or the extinction ratio is kept better than an acceptable amount. The optical signal s 1 with the signature bit pattern is carried by channel λ 1 on fiber 11, together with signals s 2 - s n . A tap monitor at some point of interest in the W DM system, noted 14 on Figure 1, taps fiber 11 and receives a fraction of signal s, which comprises a respective fraction of the optical signals s 1 - s n for all channels λ 1 - λ n . The tapped fraction is converted to an electrical signal, shown by reference numeral 17, by PIN diode 16 and then amplified by transimpedance amplifier 18. The electrical signal v at the output of transimpedance amplifier 18 comprises an ac and a dc component. The dc voltage V dc is a linear combination of the individual optical signal powers: V dc = a x P 1 + b x P 2 + c x P 3 +... n x P n where a, b, ... n are known calibration constants for the respective transmission channel, and P 1 - P n are the optical powers of the respective optical signals s 1 - s n . Signal v is filtered in a band pass filter 20 for extracting the frequencies of interest, which in this case are the component frequencies of the signature bit pattern s BP1 . A signature rms detector 22 detects the 'rms 1 ' of s BP1 independent from the interferences from the co-propagating optical signals. The signature rms detector may be realized either with an analog filter with peak detect circuitry, or with a super Nyquist sampling A/D converter and a DSP chip to implement a digital matched filter detection. At the frequencies of interest, the power of the signature bit patterns for the other channels λ 2 to λ n is practically zero, by design, so that the measured rms 1 gives P BP1 using the relation: P BP1 = rms 1 / a where 'a' is the calibration constant for channel λ 1 , known. The value of P 1 can now be determined knowing m, which gives: P 1 = rms 1 / m x a In a similar way, if the other signals s 2 -s n have their own unique signature pattern, or analog dither, the optical power of these signals can be determined in the same way, as long as all the signal dithers are designed to be independently detected from each other, for example they are frequency division multiplexed. An example of an implementation of the invention is the application of the signature bit pattern of the invention to determining the power of the bidirectional service optical channel (Bi-OSC). Bi-OSC is a service channel that is transmitted and terminated at each optical amplifier. This channel has a signalling rate of 9.72 Mb/s in each direction and is Manchester encoded in order to reduce its interference in the analog maintenance bandwidth (40 kHz) to acceptable levels. The wavelength of the channel for one direction of transmission is selected in the red band and for the reverse direction, in the blue band. The frame of the signal transmitted on this channel has 2430 bits, 96 bits being used for the signature. The average optical power of the Bi-OSC is also accounted for in order to make the average output control in the forward direction of transmission (i.e. the red band) and in the reverse direction of transmission (i.e. the blue band) more accurate by subtracting the power contribution from the respective OSC channels. The signature bit pattern for the red channel is inserted on a frame by frame basis. After the Manchester encoding, the red OSC signature pattern is inserted in the following bit positions of the red OSC frame. Red OSC signature bit pattern Bit Position Red OSC signature block 1 1111 0011 1100 0110 401 0011 1100 1111 0110 801 0011 1100 1111 0110 1217 0000 1100 0011 1001 1617 1100 0011 0000 1001 2017 1100 0011 0000 1001 The spectrum of the red OSC signature bit pattern comprises odd multiples of 4kHz, namely 4kHz, 12kHz, 20kHz, etc. which makes it phase orthogonal to the AM dithers, SONET 8kHz tones and the blue signature bit pattern. The minimum hamming distance between the first block and the other blocks within the frame is 8. The signature bit pattern for the blue channel is inserted on a two consecutive frame by two consecutive frame basis. After the Manchester encoding, the blue OSC signature pattern is inserted in the following bit positions. Blue OSC signature bit pattern Frame # Bit Position Red OSC signature block First Frame 1 1111 0011 1100 0110 401 0011 1100 1111 0110 801 0011 1100 1111 0110 1217 1100 0011 0000 1001 1617 1100 0011 0000 1001 2017 1100 0011 0000 1001 Second Frame 1 0000 1100 0011 1001 401 1100 0011 0000 1001 801 1100 0011 0000 1001 1217 0011 1100 1111 0110 1617 0011 1100 1111 0110 2017 0011 1100 1111 0110 The spectrum of the blue OSC signature bit pattern are odd multiples of 2kHz, namely 2kHz, 6kHz, 10kHz, etc. which makes it phase orthogonal to the AM dithers, SONET 8kHz tones and the blue signature bit pattern. The minimum hamming distance between the first block and the other blocks within the frame is 8. While the invention has been described with reference to particular example embodiments, further modifications and improvements which will occur to those skilled in the art, may be made within the purview of the appended claims, without departing from the scope of the invention in its broader aspect.

Claims: A method for measuring the power of an optical signal (s 1 ) travelling on a first channel (λ 1 ) of a WDM transmission system, comprising the steps of: generating a signature bit pattern (s BP1 ); adjusting the power level of said signature bit pattern to a predetermined ratio (m) with the power of said optical signal; inserting said signature bit pattern (s BP1 ) into the frame of said optical signal (s 1 ) and transmitting same along a span of transmission medium; measuring the power of said signature bit pattern (s BP1 ) at a point of interest on said span; and determining the optical power of said optical signal (s 1 ) in said point of interest. A method as claimed in claim 1, wherein said signature bit pattern (s BP1 ) is unique to said span. A method as claimed in claim 1, wherein said step of inserting comprises providing the bits of said signature bit pattern (s BP1 ) in predetermined positions of the frame of said optical signal (s 1 ) just before launching said signal on said fiber span. A method as claimed in claim 3, further comprising: providing a plurality of optical signals (s i ), each for a respective transmission channel (λ i ), where i ∈ [2, n]; and multiplexing said optical signal (s 1 ), comprising said signature bit pattern (s BP1 ), with said optical signals (s i ) into a multichannel signal (s) and launching said multichannel signal (s) on said span. A method as claimed in claim 4, wherein said step of measuring the power of said signature bit pattern (s BP1 ) comprises: taping a fraction of said multi-channel signal (s) in said point of interest; converting said fraction into an electrical signal (v); filtering said electrical signal (v) to pass a signature signal of a band comprising the frequency components of said signature bit pattern; and measuring the root mean square rms SB1 value of said signature signal. A method as claimed in claim 5, wherein said step of determining the optical power (P 1 ) of said optical signal (s 1 ) comprises applying said ratio (m) to said rms SB1 value. A method as claimed in claim 6, further comprising applying a correction factor (a) to said (P 1 ). A method as claimed in claim 5, wherein measuring the root mean square rms SB1 value of said signature signal is performed with an analog filter with peak detect circuitry. A method as claimed in claim 5, wherein said step of measuring the root mean square rms SB1 value of said signature signal comprises digital matched filter detection. A method as claimed in claim 9, wherein said digital matched filter detection is performed with a super Nyquist sampling analog-to-digital converter and a digital signal processing unit. A method as claimed in claim 4, wherein each of said optical signals (s i ) comprises a respective signature bit pattern (s BPi ) for detecting a respective optical power (P i ) of a restrictive optical signal (s i ) in said point of interest

Inventors: HABEL RICHARD ACHILLE, HARLEY JAMES, HABEL, RICHARD ACHILLE, HARLEY, JAMES
Applicants: NORTEL NETWORKS LTD, NORTEL NETWORKS LIMITED
Application year: 1998
Main classification: H04B10
IPC codes: H04B10, G01J1, H04J14