GB2430569A

GB2430569A - Passing a modulated multi-wavelength optical signal through a dispersive single mode fibre to produce a test signal

Info

Publication number: GB2430569A
Application number: GB0519617A
Authority: GB
Inventors: Giampaolo Bendelli; Mario Puleo
Original assignee: Agilent Technologies Inc
Current assignee: Agilent Technologies Inc
Priority date: 2005-09-27
Filing date: 2005-09-27
Publication date: 2007-03-28
Also published as: GB0519617D0

Abstract

The present invention concerns an optical signal generator. The signals produced by the generator may be used in comprehensive stressed receiver sensitivity testing of receivers for optical fibre communication. The present invention relies largely on optical components rather than electrical components, as in prior art systems. Each one of a plurality of optical sources generates an optical signal. Each optical signal has a different wavelength to any of the other optical signals. The optical signals are multiplexed in combiner 22 and the resulting multi-wavelength signal is modulated by an electro-optic modulator 24. The modulated signal is then passed through a length of dispersive single mode optical fibre 26, to produce differential delays between different wavelengths in the modulated multi-wavelength signals, and thus separate the pulses in time.

Description

- 1 2430569 "Optical signal generator and method" ***

TEXT OF DESCRIPTION

The invention relates to optical signal generators.

The invention was developed with specific attention paid to a possible use in comprehensive stressed receiver sensitivity testing of receivers for optical fibre communications such as 1OGbE LRM (10 Gb Ethernet over multimode fibres with electronic dispersion compensation) modules/receivers.

Generating an optical signal suitable for comprehensive stressed sensitivity testing of such receivers forms the subject matter of e.g. IEEE8O2.3aq draft 2.2 (draft 2.2 being at present an intermediate draft) guideline. The aim pursued is essentially to simulate in a controlled way the propagation in a multirnode fibre link.

The solution suggested by the IEEE8O2.3aq draft guideline essentially relies on multipath propagation of lOGbit/s electrical signals, which are then converted into the optical domain. The block diagram of figure 1 is exemplary of a possible conventional implementation of the principle proposed in the captioned draft guideline.

In the block diagram of figure 1, reference 10 indicates a high speed (e. g. 10 Gbit/s) digital symbol pattern generator to which a Gaussian low pass filter 12 (such as a 47 ps rise filter) is cascaded to convert the digital symbol pattern produced by the generator 10 into a pattern of "actual" pulse signals.

Reference numeral 14 denotes a summation node into which a signal representative of Gaussian noise produced by a generator 16 is added to the signal pattern from the filter 12 to produce a signal affected by noise with a given S/N (signal-to-noise) ratio.

The signal from the summation node 14 is injected into an Inter Symbol Interference (ISI) generator 18.

The incoming signal to the ISI generator 18 is split via a RF splitter 180 over of a plurality of signal propagation paths (to the number of four in the exemplary embodiment shown herein) to be subjected to respective different delays and attenuation values.

Each signal propagation path in the ISI generator 18 includes a delay element 181 followed by a variable attenuator 182. The delayed and attenuated signals over the various propagations paths are then recombined at an output node 183 (typically a RF combiner) to be fed to linear electro/optical converter 20.

The optical signal from the converter 20 represents the desired test signal to be fed to the device/apparatus under test (typically an optical receiver - not shown) Arrangements such as the exemplary arrangement shown in figure 1 are affected by a number of crosstalk, reflection and linearity issues.

More specifically, the RF splitter 180 and combiner 183 are not completely isolating. As a consequence, reflected signals from one path leak undesirably into other paths, thus giving rise to crosstalk.

The arrangement shown in figure 1 also inherently exhibits a high insertion loss, which requires to be countered via broadband linear amplification. This is difficult to achieve in a truly satisfactory manner, especially at bit rates as high as lOGbit/s.

Additionally, variable amplifiers/attenuators are exposed to changes in their frequency responses depending on setting, and a converter 20 that is truly linear over a wide amplitude range is difficult to implement.

The need therefore exists for alternative arrangements adapted to generate an optical signal suitable for comprehensive stress sensitivity testing (such as dictated e.g. by the IEEE8O2.3aq draft standard), which dispense with the intrinsic drawbacks exhibited by the conventional implementation discussed in the foregoing with reference to figure 1.

According to the present invention that object is achieved by means of an optical signal generator having the features set forth in the claims that follow. The invention also relates to a corresponding method. The claims are an integral part of the disclosure of the invention provided herein.

In brief, a preferred embodiment of the arrangement described herein includes: - a plurality of optical sources each emitting an optical signal at a respective wavelength, - a combiner and modulator arrangement to produce from said optical signals from said plurality of optical sources a multi-wavelength optical signal modulated with an electrical modulation signal, and - a length of dispersive single mode optical fibre for propagating said modulated multi-wavelength optical signal to produce a propagated multi- wavelength optical signal wherein each wavelength in said propagated multi-wavelength optical signal is subjected to a respective propagation delay, thus producing differential delays between different wavelengths in said propagated multi-wavelength optical signal.

In such an arrangement, the different propagation delays that are typical of multipath propagation are obtained by propagating in a dispersive single-mode fibre optical signals that carry the same data stream at different wavelengths. These optical signals do not interfere between them, and the adjustment of amplitude and delay characteristics of each signal can be easily obtained by acting on quasi-static (i.e. slowly varying) variables such as the temperature and current of a laser source.

In a preferred embodiment of the arrangement described herein, multipath propagation of a signal (e.g. a lOGbit/s signal) is obtained by simultaneously modulating a plurality of continuous wave (CW) light sources such as laser sources having different emission wavelengths via a shared external modulator driven by an electrical data stream. Differential delays between paths result from propagation of the signal at different wavelengths in a single mode fibre exhibiting chromatic dispersion. These differential delays are obtained as a product of the wavelength difference of the various signals times the chromatic dispersion of the fibre. Different levels of amplitude (attenuation) can be simply obtained by setting proper power levels for the various CW laser sources. Fine tuning of delay can be achieved by setting the laser emission wavelengths via the laser temperature control.

The arrangement described herein is fully compliant with the principle of operation prescribed by the IEEE8O2.3aq draft 2.2. The arrangement described herein is completely exempt from the disadvantages and drawbacks described previously in connection with the conventional implementation schematically represented in figure 1.

The invention will now be described, by way of example only, with reference to the annexed figures of drawing, wherein: - Figure 1 is exemplary of a conventional implementation, as already been described in the foregoing; - Figure 2 is a block diagram exemplary of a possible implementation of the arrangement described herein; and - Figure 3 is a schematic diagram representative of signals generated with the arrangement illustrated in figure 2.

In the block diagram of figure 2, reference numerals 10, 12, 14 and 16 are again representative of a generator of a high speed (e.g. lOGbit/s) digital symbol pattern, a filter to convert the pattern of digital symbols produced by the generator 10 into a corresponding pattern of "actual" signals, a summation node and a Gaussian noise generator to produce a noise signal to be summed at the node 14 with the signal pattern from the filter 12.

In the block diagram of figure 2, reference 20 is representative as a whole of a plurality of continuous wave (OW) single frequency laser sources of any known type.

In order to provide a direct comparison with the conventional arrangement of figure 1, the laser sources 20 are to the number of four, but the plurality of sources 20 can notionally include any number n of sources. These sources are assumed to be approximately equally wavelength spaced laser sources producing optical radiations at wavelengths designated A, X2, A3, and A4, respectively.

Those of skill in the art will appreciate that terms such as "light", "optical" and so on are used herein according to the established use in the area of optical communication and integrated optics. As such, these terms are in no way limited to visible light radiation but include, e.g. optical radiation in the infrared (IR) and ultraviolet (UV) ranges.

The optical radiation at different wavelengths emitted from the various sources 20 are combined in combiner 22 such as a combiner of the type currently used in wavelength division multiplex (WDM) optical fibre communications systems.

The combined multi-wavelength radiation from the combiner 22 is fed to an electro-optical modulator 24 (again of a known type, such as of current use in optical fibre communications systems) to be modulated by means of the signal (with added noise) from the summation node 14.

Those of skill in the art will appreciate that the modulating signal being with added noise, is a feature of interest for the specific use considered herein.

However this does in no way represent a mandatory feature of the arrangement of the invention, which means that the noise generator 16 and the summation node 14 do not represent essential elements of the invention.

Additionally, it will be appreciated that the arrangement disclosed, with the modulator 24 cascaded to (i.e. downstream of) the combiner 22, is advantageous over a possible alternative embodiment where the optical signals from the various sources 20 are first modulated, by means of separate optical modulators, with the electrical modulation signal from the node 14 and then combined.

Whatever the arrangement used to produce it, the modulated multiwavelength (i.e. combined) optical signal thus obtained is propagated over a length of dispersive single-mode optical fibre, designated 26.

The length of fibre in question may include e.g. 1 km of standard G652 (e. g. SMF28) single mode fibre.

As a result of propagation over the length of optical fibre 26, each component of the modulated multi-wavelength optical signal (i.e. each of the components at the various wavelengths A1, A2, A3, and A) will experience a different propagation delay.

This is according to well-known principles of propagation of optical signals over dispersive single mode optical fibres, which do not require to be discussed in detail here: for a general discussion of this phenomenon see e.g. Fiber Optics Communications Handbook", TAB professional and reference books 1990.

The differences between the various propagation delays, that is the differential delays between the various components (A1, A2, A3, and A4) will be dictated by the product of the wavelength difference (e.g. A1- A2, A-A3, A3-A4 and so on) times the chromatic dispersion of the fibre (e. g. typically around 16-18 ps/nm km in the 1550 nm operating window) A wavelength spacing of approximately 4 nm in conjunction with a fibre chromatic dispersion of about 18 ps/nm km (i.e. about 1 km of SMF28 fibre) will produce a 75 ps differential path delay as specified by the draft standard referred to in the foregoing. The signal propagated over the length of fibre 26 will thus exhibit the same desired characteristics of the signal from the electro-optical converter 20 of the block diagram of figure 1. However the signal propagated over the length of fibre 26 will not be affected by any of the disadvantages/drawbacks discussed in the foregoing.

Specifically, the signals propagated over the fibre 26 at different wavelengths will not interfere between them and will thus be exempt from crosstalk.

The attenuation due to propagation over the length of fibre 26 can be as low as 0.5 dB, so that no need for amplification arises in the arrangement of figure 2.

The electro-optical converter, difficult to provide with a truly linear behaviour over a wide amplitude range, is placed in the arrangement of figure 2 in a position where the driving signal amplitude is constant and can be optimized for better signal linearity.

Additionally, the relative levels of various components propagated at the different wavelengths can be simply adjusted by setting the output power level of the respective laser sources included in the plurality 20. This adjustment is easily obtained by acting on a "static" variable (essentially the laser drive current) Controlling the junction temperature of each laser source in the plurality 20 permits to precisely set the respective emission wavelength and thus to control with a high degree accuracy the differential delays obtained after propagation over the length of fibre 26, thus compensating for the small chromatic dispersion differences between wavelengths. As is well known, the chromatic dispersion of optical fibres, such as the fibre 26, is also generally stable and hardly affected by any ambient parameter within the reasonable limits experienced during testing.

The time diagrams of figure 3 are related to an experimental arrangement as illustrated in figure 2 and including for laser sources 20 emitting at respective wavelengths spaced 4 nm from each other.

Specifically, the lower traces designated 101, 102, 103, 104 are representative of individual signal components propagated over a length of fibre 26 including one km of single mode optical fibre at wavelengths A1, A2, A3, and A4 respectively equal to 1550,1554,1558,1562 nm. The four traces/signals in question clearly exhibit the same modulation pattern with different propagation delays (the traces 102, 103 and 104 being increasingly delayed with respect to the trace 101) while e.g. the signal 103 exhibits a higher attenuation than the other three signals 101, 102 and 104.

The trace 110 is representative of the cumulative multi-wavelength output signal from the fibre obtained by adding the various signals 101, 102, 103 and 104 considered in the foregoing.

The same operating principle described in the foregoing can be implemented at wavelengths around 1310 nm, by using laser sources in this window in conjunction with an optical fibre with suitable chromatic dispersion at 1310 nm, e.g. a G653 (DSF) fibre.

Consequently, without prejudice to the underlying principles of the invention, the embodiments and details may vary, also significantly, with respect to what has been described and illustrated by way of example only without departing from the scope of the invention as defined in the annexed claims.

Claims

- 10 - CLAIMS

1. An optical signal generator including: - a plurality of optical sources (20) each emitting an optical signal at a respective wavelength (A1, A2, A3, A,1), - a combiner (22) and modulator (24) arrangement to produce from the optical signais. from said plurality of optical sources (20) a multiwavelength optical signal modulated with an electrical modulation signal, and - a length (26) of dispersive single mode optical fibre for propagating said modulated multi-wavelength optical signal to produce a propagated multi-wavelength optical signal wherein each wavelength in said propagated multi-wavelength optical signal is subjected to a respective.. propagation delay, thus producing differential delays between different wavelengths in said propagated multi-wavelength optical signal.

2. The generator of claim 1, characterised in that said combiner (22) and modulator (24) arrangement includes: - a combiner (22) to combine the optical signals from said plurality of optical sources (20) to produce a combined multi-wavelength optical signal, and - a modulator (24) to modulate said combined multi-wavelength optical signal with said electrical modulation signal to produce said modulated multi- wavelength optical signal.

3. The generator of either of claims 1 or 2, characterised in that said plurality of optical sources (20) are continuous wave (CW) optical sources.

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4. The generator of any of claims 1 to 3, characterised in that said plurality of optical sources (20) are laser sources.

5. The generator of any of the previous claims, characterised in that said plurality of optical sources (20) have adjustable emission power levels, whereby the amplitudes of the wavelengths in said propagated multiwavelength optical signal are selectively adjustable.

6. The generator of any of the previous claims, characterised in that said plurality of optical sources (20) emit optical signals at approximately equally spaced wavelengths (A1, A2, A3, A4)

7. The generator of claim 6, characterised in that said approximately equally spaced wavelengths (A1, A2, A3, A4) have wavelength spacings of about 4 nm the rebetween.

8. The generator of any of the previous claims, characterised in that it includes a modulation source (10, 12, 14, 16) to produce said electrical modulation signal.

9. The generator of claim 8, characterised in that said modulation source (10, 12, 14, 16) includes a noise source (16) to produce a noise component of said electrical modulation signal.

10. The generator of either of claims 8 or 9, characterised in that said modulation source (10, 12, 14, 16) includes: - a source (10) of a pattern of digital symbols, and - 12 - - a filter (12) to convert said pattern of digital symbols into a pattern of digital signals to produce said electrical modulation signal.

11. The generator of any of claims 8 to 10, characterised in that said modulation source (10, 12, 14, 16) is a high frequency modulation source, preferably in the Gbit/s range.

12. A method of generating optical signals including the steps of: producing (20) a plurality of optical signals at respective wavelengths (A1, A2, A3, A4), - combining (22) and modulating (24) said plurality of optical signals to produce a multi- wavelength optical signal modulated with an electrical modulation signal, and - propagating said modulated multi-wavelength optical signal over a length (26) of dispersive single mode optical fibre to produce a propagated multi- wavelength optical signal wherein each wavelength in said propagated multi-wavelength optical signal is subjected to a respective propagation delay, thus producing differential delays between different wavelengths in said propagated multi-wavelength optical signal.

13. The method of claim 12, characterised in that it includes the steps of: - combining (22) said plurality of optical signals to produce a combined multi-wavelength optical signal, - modulating (24) said combined multi-wavelength optical signal with said electrical modulation signal to produce said modulated multi-wavelength optical signal.

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14. The method of either of claims 12 or 13, characterised in that it includes the step of producing (20) said plurality of optical signals at respective wavelengths (A1, A2, A3, A4) as continuous wave (CW) optical signals.

15. The method of any of claims 12 to 14, characterised in that it includes the step of producing said plurality of optical signals via laser sources (20)

16. The method of any of the previous claims 12 to 15, characterised in that it includes the step of selectively adjusting the power levels of said plurality of optical signals, thereby adjusting the amplitudes of the wavelengths in said propagated multi- wavelength optical signal.

17. The method of any of the previous claims 12 to 16, characterised in that said plurality of optical signals have approximately equally spaced wavelengths (A1, A2, A3, A4)

18. The method of claim 17, characterised in that said plurality of optical signals have wavelength spacings of about 4 nm therebetween.

19. The method of any of the previous claims 12 to 18, characterised in that it includes the step of adding (14, 16) a noise component to said electrical modulation signal.

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20. The method of any of claims 12 to 19, characterised in that it includes the step of generating said electrical modulation signal by: generating (10) a pattern of digital symbols, and - converting (12) said pattern of digital symbols into a pattern of digital signals constituting said electrical modulation signal.

21. The method of any of claims 12 to 20, characterised in that it includes the step of generating said electrical modulation signal as a high frequency modulation signal.

22. The method of any of claims 12 to 21, characterised in that it includes the step of generating said electrical modulation signal as a frequency modulation signal in the Gbit/s range.

23. An optical signal generator, substantially as described and illustrated and for the purposes herein specified.

24. A method of generating optical signals, substantially as described and illustrated and for the purposes herein specified.