Technical Area The invention concerns a system for obtaining the characteristic spectral information of light emitted from an optical communication fibre, light source or other component used in telecommunication.

The Prior Art Optical communication fibres are achieving an ever-increasing significance as carriers of information. Recent rapid technological developments both in fibre optics and in electro-optical components for WDM (wavelength division multiplexing) and for DWDM (dense wavelength division multiplexing) have involved the necessity of ever-improving instrumentation for the monitoring of these components, of the optical signals that the fibres carry and of the properties of the fibres. The said monitoring may take place on-line, that is, directly in association with reception or transmission of the optical signals. The monitoring is significant since the signals, after having been carried over distances of up to several thousand kilometres, may be deformed during the transmission if, for example, the fibres do not satisfy the demands placed on them.

The light that carries signals in fibre communication is light in the near infrared (NIR) region of wavelengths. Only certain wavelength bands within this region are of significance, namely 1,200-1,700 nm for what are known as"single-mode"fibres, and 800-900 nm for what are known as"multimode fibres". Two bands, namely 1,200-1,400 nm and 1,500-1,700 nm, are most relevant within the first band, since signals within these bands can be carried over long distances and amplified with the aid of what are known as "active fibres".

It is a current trend to use several wavelengths, up to several hundred, to carry information within these bands, since the capacity of the fibre can in this way be increased.

A separation between wavelength channels of as small as 1 nm has been discussed. This places strict requirements on the instruments that spectrally monitor and characterise the signal light. The most important requirements are: 1. High precision when measuring the channel wavelengths and their profiles, 2. High spectral resolving power, 3. The ability to record rapidly and simultaneously all channel wavelengths in the said band.

These requirements have been met by the use of not only wavelength selective spectral equipment but also interferometric spectral equipment. While it is true that the former type, which most often exploits a diffraction grating, can display a high spectral resolving power and accurate determination of channel wavelengths, it can seldom allow the measurement of more than one channel wavelength at a time.

This is also the case for interferometric spectral equipment, which, although it is true that this equipment can measure the complete said band, is burdened with the major disadvantage that the measurement is carried out sequentially and thus becomes time- consuming.

Those instruments that are able to measure more than one wavelength at a time employ spectrographs in combination with what are known as"detector arrays", that is, detectors that contain several sensor elements placed in a line. Such a device is described in PCT WO 99/67609. In this document, the various channel wavelengths are each assigned to a sensor element. While it is true that the simultaneous intensities of channel wavelengths can be measured with this device, it is not possible either to determine the exact value of the channel wavelengths or to measure their intensity profiles.

Summary of the Invention The present invention eliminates the said limitations of the wavelength-selective and the interferometric spectral devices by making possible the spectral measurement of the light from communication fibres, satisfying all of the three requirements listed above: namely high precision of measurement of the channel wavelengths, high spectral resolving power and the ability to measure the intensities, wavelength values and intensity profiles of the channel wavelengths simultaneously.

The invention concerns a system for the characterisation of the said light, which contains a connection to a communication fibre from the end of which light is led to an optical spectrograph comprising a collimating optical means, at least one diffraction grating that, in the wavelength band 800 nm-1, 700 nm or parts thereof, spreads the wavelength components of the light in one plane of spreading by diffraction into several orders of diffraction, a wavelength-dispersing means, from the group of prisms and diffraction gratings, that in the said wavelength band spreads the wavelength components of the light in a second plane of spreading that is approximately perpendicular to the first plane of spreading, a focusing means that achieves the imaging of the said fibre-end at each wavelength in the said wavelength band onto the sensor surface of a digital camera that

subsequently records and converts the said images into digital electrical signals that are transferred to a computer, which computer converts the signals into a spectrum that is characteristic for the light from the fibre and that calculates from this spectrum magnitudes that are characteristic for the fibre, the light source that illuminates the fibre, and/or optoelectrical components that are used in telecommunication.

Other characteristic properties of the invention are made clear by the accompanying claims.

Description of Embodiments The system according to the invention will be described in more detail below with the aid of drawings that show embodiments.

Figure 1 shows a system according to the invention with a spectrograph that achieves a two-dimensional spectrum that is recorded with a digital camera.

Figure 2 shows an embodiment according to the invention similar to the one in Figure 1, the difference being that the pathway of light through the spectrograph is different.

In order to be able to describe the invention more easily, mathematical expressions are reproduced below that describe diffraction in a grating. The principal properties of diffraction that are relevant for the invention are made clear by what is known as the "grating equation", which is as follows: The expression is valid when the incident light is parallel to what is known as the "plane of spreading"of the grating, which is that plane that is perpendicular to the rulings of the grating and that contains the normal to the surface of the grating.

The symbols included in the equation denote the following quantities: d = the separation of rulings on the optical surface of the diffraction grating a = the angle between the direction of the incident light and the normal to the surface of the grating ß = the angle of diffraction, or the angle between the direction of the exit light after diffraction and the normal to the surface of the grating M = an integer specifying what is known as the"order"of the spectrum and X = the wavelength of the light.

The angle of incidence,. a, and the grating constant, d, are constants when using a diffraction grating, while the angle of diffraction, ß, thus depends on the wavelength, B.

This is described by the function F (p) which thus describes the spectrum that is formed. On the other hand, one spectrum arises for each value of the integer M, which means that the grating generates several spectra, with each sub-spectrum being associated with a certain value of the integer M. Consequently, each sub-spectrum is denoted by its integer value as follows:"the Mth order spectrum". If 4 denotes the wavelength that is diffracted into the spectrum of order M, the equation above shows that all wavelengths XK that are diffracted into a spectrum of order K satisfying the condition: exit from the diffraction grating in the same direction. These wavelengths will be imaged on top of each other in one focal plane. This is known as"overlapping"and is a property of diffraction gratings in which the grating constant d is significantly greater than the wavelength B. These gratings, normally termed"Echelle gratings"or"Echellette gratings" are usually provided with rulings that have a profile that approaches that of a staircase, whereby the step-surface of each ruling is plane and reflecting. An Echelle grating concentrates the intensity of the diffracted light into a certain direction, which is determined by the orientation of the said step-surface. Further properties of these gratings are made clear by, for example, the"Diffraction Grating Handbook", Richardson Grating Laboratory, 2000.

In the device according to the invention shown in Figure 1, (10) denotes a communication fibre of known type, from the end (18) of which the light that is to be characterised is emitted. The fibre is connected to a holder (11) of known design, in which the position of the end of the fibre is held fixed. The light from the end (18) of the fibre subsequently impinges upon a collimator (12), which is an optically imaging means placed such that the end (18) of the fibre is located in the putative focal point of the collimator.

The route taken by the light through the system according to the invention, that is, its ray pathway (20), is specified in Figure 1 by a dashed line with arrowheads. The collimator (12) may comprise, for example, one or several lenses or mirrors. A plane diffraction grating (13), which in the example shown is of the Echelle type, is placed in the ray pathway after the collimator. This, thus, diffracts the light into high spectral orders (M) whereby most of the diffracted energy impinges a wavelength-dispersing means, in this case a diffraction grating (14). The light in the said wavelength band is diffracted in the embodiment into spectra whose orders are in excess of 10. The rulings of the diffraction grating (13) have a ruling density that is lower than 100 rulings per millimetre.

The diffraction grating (14) is so oriented that its plane of spreading acts essentially perpendicular to the plane of spreading that is defined by the diffraction grating (13). The diffraction grating (14), known as an"order sorter", diffracts the light into low spectral orders, typically of order 1 or 2. This is in contrast to the diffraction grating (13).

The light that has thus been diffracted twice impinges thereafter upon the focussing optical means (15) which, in the same way as the collimator (12), may comprise one or more lenses or mirrors. The means (15) images the light from the end (18) of the fibre onto a detector (16) in a digital camera (17).

The detector (16) is of a two-dimensional type, which means that it comprises picture elements that are organised both into rows and into columns. The imaging that the focussing means (15) produces on the detector (16) will be, through the collaboration of the two diffraction gratings (13) and (14), a two-dimensional spectrum, built up from elements in rows underneath each other. Each segment, which represents the spectrum in a spectral order from the diffraction grating (13), is a sub-spectrum of the complete spectrum that is formed by all segments together. This is achieved by choosing in a known way the properties of the order sorter (14) and the diffraction grating (13) such that no wavelength within the intended wavelength region of the means (15) is imaged outside of the surface of the detector (16). This constitutes a guarantee for what is known as"spectral continuity".

The digital camera (17) converts the intensity value in each picture element into a digital signal that is stored in the memory of the computer (19). An image processing program converts the digital image information into a spectrum consisting of a sequence of numerical values arranged in pairs, in which one value represents the wavelength of the light (alternatively converted into the frequency of the light), and the other value in the said pair represents the spectral intensity or power of the light expressed in a unit that is suitable for the application, such as dB, mW, dBm. The wavelength of the light is most often measured in nm and its frequency in THz. One skilled in the arts can, in a known manner, calculate on the basis of this sequence relevant magnitudes of the light from the fibre (10) that are characteristic of the light-conducting properties of optical telecommunication fibres, of light sources (21), and/or of other optical telecommunication components.

One alternative embodiment according to the invention is shown in Figure 2. In this figure the fibre (10) and its end (18), the holder (11), the ray pathway (20), the collimator (12), the wavelength-dispersive means, that is, the order sorter (14), the diffraction grating (13), the focussing means (15), the detector (16) in the digital camera (17) and finally the

computer (19) containing the above-mentioned program are all again present. However, the order sorter (14) in Figure 2 is constituted by a prism, with the help of which a separation into the spectral segments can be obtained in the same way as that of the grating (14) in Figure 1. The prism (14) is in the embodiment shown of the type known as"Littrow".

Furthermore, Figure 2 shows an embodiment in which the sequence of the grating (13) and the order sorter (14) in the ray pathway (20) is the inverse of that shown in Figure 1, which means that the order sorter (14) in Figure 2 is placed before the grating (13) in the ray pathway (20).

Figure 2 also illustrates a further embodiment according to the invention. This demonstrates a cylindrical lens (22) placed in the ray pathway (20). Images of the light from the end (18) of the fibre can, with the aid of the lens, be spread out into an image in the form of a straight line that is oriented approximately perpendicular to the spectral segments in the focal plane. This lens achieves a spreading of the light from the fibre out over several of the picture elements of the detector (16). The values read from these picture elements are summed in the program in the computer (19) to give an intensity value. Since this is built up from the measured values of several of the picture elements, the lens involves an increase in the measuring area for the intensity values in the above-mentioned sequence that is achieved by the program in the computer.

The system can, according to the invention, be optimised for one or several of the most common wavelength bands, namely that known as the O-band (Original) 1,260- 1,360 nm, that known as the E-band (Extended) 1,360-1,460 nm, that known as the S- band (Short wavelength) 1,460-1,530 nm, that known as the C-band (Conventional) 1,530 - 1, 565 nm, that known as the L-band (Long wavelength) 1,565-1,625 and finally, that known as the U-band (Ultra-long wavelength) 1,625-1,675 nm.

The system according to the invention makes possible rapid measurement and calculations of the said magnitudes, up to one hundred times per second, whereby the rate is limited only by the speed of the digital camera and of the computer.

It is particularly significant in telecommunication to be able to calculate from the spectral information not only the magnitudes that are characteristic for the properties of the fibre, but also magnitudes that characterise the properties of the light source. The light source (21), which is often a diode, diode laser, laser diode or other electro-optical component, has certain properties that must be determined. These include the wavelength, frequency, channel width, channel location, channel separation and SMSR (the"side mode

suppression ratio"), which involves the relationship between the primary intensity of a channel wavelength and the intensity of a interfering side channel or side mode. All of these magnitudes can be determined using the present invention. In the case in which it is desired to measure the properties of the fibres, the light source (21) can be constituted by, in addition to the types listed above, a light source that emits white light, such as a lamp of the halogen type. The characteristic properties include the spectral attenuation, what is known as the"cut off'wavelength, and various types of dispersion.

The detector (16) can be according to the invention one of three types, namely of the CCD (charge-coupled device) type, the InGaAs (indium-gallium-arsenide) type or the MCT-HgCdTe (mercury-cadmium-tellurium) type.

Furthermore, the system according to the invention can be optimised such that it measures only one of the sub-regions in the region 800-1,700 mm, namely the sub-region 800-900 nm, the sub-region 1,200-1,700 nm, the sub-region 1,200-1,400 nm or the sub- region 1,500-1, 700 nm.

The invention is not limited to the revealed embodiments of the same. Thus it can be arbitrarily varied within the field of the attached claims. In this way, for example, the collimator (12) can be constituted by a concave mirror, or by a combination of concave mirrors and lenses. The same is true for the focussing means (15), which can be constituted by a concave mirror or by a combination of concave mirrors and lenses. Furthermore, the cylindrical lens (22) can, for example, be constituted by a mirror with a cylindrical surface shape. Other optical components may also be present in the ray pathway (20), selected from the group of lenses and mirrors, in addition to those shown in the described embodiments. Finally, the diffraction gratings (13) and (14) may be constituted by other types of diffraction grating such as, for example, transmission gratings.