Patent Description:
Fibre to the Premises (FTTP) provides optical fibre all the way from the exchange or switching centre to a customer's premises. At the customer end of the fibre, which is commonly cut to length at the customer premises, an optical connector is required to terminate the fibre in order to facilitate connection into the customer's premises equipment. The desire to quickly connect the customer's premises equipment has led to the introduction of a field-fit connector, which involves a mechanical method of placing a connector on the freshly cut end of the fibre. As the name suggests, the connectors are fitted in-situ, so quality assurance of the connection is problematic when compared to, say, optical assemblies created in a manufacturing facility. Field-fit connectors (also known as field-installable connectors or field-assembly connectors) are mechanical in nature and can be prone to faults relating to fitting of the connectors to the fibre and the finishing of the fibre itself that can adversely affect service. For example, quality issues with field-fit connectors can introduce excessive losses leading either to no service continuity necessitating rework or to a degradation of the service provided to the customer over time leading to the need for restorative work during the lifetime of the connection. Unlike factory-fitted fusion-splice connections, the lack of a reliable confirmation system for field-fit connecters, means that faults with them can be difficult to detect. There is therefore a need for an improved method for reliably checking that a field-fit connector has been properly fitted to the fibre.

Field-fit connector loss relates to losses arising from the quality of the fibre joint created when the field-fit connectors is mated with a second optical fibre e.g. through a second connector. Losses may be caused by several factors, including gaps between the cut end of the fibre and the end of an optical fibre in a mating connector, misalignment of the fibre within the connector and where the cut end of the fibre is not properly cleaned and polished. One way to detect a quality issue with field-fit connecters, is to measure the optical loss through the connector after fitting to the fibre. However, it can be difficult to get an accurate measurement of the optical loss of the connector in the field, i.e. once fitted to the fibre at the customer's premises. One way to do this is to take readings of optical power in light received from the network with a view to quantifying the loss introduced by the connector. This works by taking a first reading at the distribution point of optical power in light received from the network without the drop fibre and then taking a second reading at the customer premises of optical power in light received from the network taken after the drop fibre and connector has been fitted. From a comparison of the two optical power readings, the loss introduced by fitting the connector can then be estimated (e.g. as the loss in the drop fibre will also contribute to the second power reading). However, this requires action at both the distribution point and the customer premises and can be time-consuming, especially where the distribution point is located remote from the customer premises.

Current techniques for fitting and testing a fibre from a connectorized distribution point to the customer's premises in FTTP are represented in <FIG>. <FIG> shows, a conventional distribution point <NUM> (e.g. located at a telegraph pole, a buried junction box or PCP) that is the closest point with suitable connectors in access network <NUM> to the customer's end <NUM> of drop fibre <NUM>. Light <NUM> from the network (e.g. light from a local switching centre - not shown) passes along fibre <NUM> to the distribution point <NUM>. At the distribution point <NUM>, the fibre <NUM> from the network is terminated in a factory-fit connector <NUM> suitable for connection via a second factory- or field-fit connector <NUM> to the drop fibre <NUM> to the customer's premises <NUM>. The customer's end <NUM> of drop fibre <NUM> is provided with a field-fit connector <NUM>. Connector loss measurement will now be described with reference to <FIG>. As shown in <FIG>, before the drop fibre <NUM> is connected, an optical power meter <NUM> is connected via a patch cord <NUM> fitted with a factory-fit connector <NUM> to the distribution point <NUM>, i.e. in place of the drop fibre <NUM>. The optical power meter <NUM> is connected via patch cord <NUM> and connectors <NUM> and <NUM> at the distribution point <NUM> to the fibre <NUM> from the network. The optical power meter <NUM> allows measurement of light from the network (e.g. light from the local switching centre - not shown). For example a typical optical power reading at this point may be -15dBm.

Once the power measurement is complete, the patch cord and power meter can be disconnected and the drop fibre <NUM> is connected (as shown in <FIG>) to provide service to the customer's premises. <FIG> shows the distribution point, with the patch cord <NUM> and optical power meter <NUM> removed. In their place, is connected the drop fibre <NUM> running to the customer's premises. Light from the network can now propagate to the end of the fibre at the customer's premises.

As shown in <FIG>, a power meter <NUM> is attached to the field-fit connector <NUM> at the customer's premises. The optical power received from the network (for example, from the exchange or switching centre) at the field-fit connector can then be measured. A typical optical power reading at this point may be -17dBm. From the difference in power levels (i.e. -<NUM> - (-<NUM>)) the loss (2dB in this example) introduced by the combination of the field-fit connector <NUM> and drop fibre <NUM> may be determined. <CIT> discloses an optical waveguide grating interrogation system comprising bi-directional optical amplifying and gating means. <CIT> relates to the calibration of a system for determining an optical loss of a device under test. <CIT> discloses a method for assessing the quality of an optical fibre cleaved end-face. <CIT> discloses a system and method for increasing the fiber coupling efficiency of diode lasers.

According to the invention, there is provided a method of quantifying loss associated with an optical connector that is connected to an end of an optical fibre; in which the optical fibre comprises a plurality of embedded optical reflectors distributed periodically along the length of the fibre;
in which the method comprises: inserting an optical signal into the fibre through the optical connector; measuring a component of the optical signal reflected by at least one of the plurality of embedded optical reflectors, in which the component is received through the optical connector; calculating the difference in power level between the inserted and reflected signals; and quantifying, based on the calculated power level difference and the reflectivity of the embedded optical reflectors, the loss associated with the optical connector.

In this way, the invention provides a method of calculating losses more accurately and more conveniently allowing field engineers to verify the correct installation of the field-fit connector quickly with more effectively. The invention reduces process steps for installing a drop fibre to customer premises, for example, by not requiring any action at the exchange or at any intermediate location along the fibre.

According to an embodiment, the optical fibre connects a switching centre and a customer premises; in which the end of the optical fibre to which the optical connector is connected is located at the customer premises.

According to an embodiment, the method comprises cutting the optical fibre to length at a point located between the at least one of the plurality of embedded optical reflectors and a second one of the plurality of embedded optical reflectors and fitting the connector to the end of the fibre.

According to an embodiment, the plurality of embedded optical reflectors are configured to reflect light at the same wavelength.

According to an embodiment, the at least one of the plurality of embedded optical reflectors comprises the optical reflector closest along the fibre to the optical connector.

According to an embodiment, each embedded optical reflector comprises a fibre Bragg grating.

According to an embodiment, the method comprises attaching a test equipment to the optical connector; in which the test equipment comprises a source of the optical signal, an interface configured to insert the optical signal into the fibre through the optical connector and configured to receive the component of the optical signal from the fibre through the optical connector, and a detector to detect the power of the received component of the optical signal.

According to an embodiment, the method comprises:.

According to an embodiment, not covered by the present invention, there is provided an optical fibre comprising a plurality of embedded optical reflectors distributed periodically along the length of the fibre, wherein an optical connector is connected to an end of the optical fibre.

In this way, the optical fibre enables a simplified and more efficient method of measuring losses, allowing field engineers to verify the correct installation of a field-fit connector quickly with more effectively. The optical fibre reduces process steps for installing a drop fibre to customer premises, for example, by not requiring any action at the exchange or at any intermediate location along the fibre.

According to an embodiment, not covered by the present invention, the embedded optical reflectors are fibre Bragg gratings.

According to an embodiment, not covered by the present invention, the plurality of embedded optical reflectors are configured to reflect light at the same wavelength. According to an embodiment, not covered by the present invention, the location along the fibre of each of the plurality of embedded optical reflectors is marked externally. The optical connector and/or optical fibre may be for use in the field of optical telecommunications.

The fibre may contain <NUM> or more reflectors, may contain <NUM> or more reflectors and preferably contains <NUM> or more reflectors. Reflectors may be present along more than half of the length of the fibre. Reflectors may be present along substantially the whole length of the fibre. The separation of the reflectors may be more than <NUM>. The separation of the reflectors may be between greater than or equal to <NUM> metres and less than or equal to <NUM> metres. The separation of the reflectors is preferably greater than or equal to <NUM> metres and less than or equal to <NUM> metres.

According to an embodiment, not covered by the present invention, there is provided a drop cable for supply of communication services to a customer; in which the drop cable comprises the optical fibre. According to an embodiment, not covered by the present invention, the cable connects a switching centre and a customer premises.

As shown in <FIG>, according to the embodiment, the distribution point <NUM> is connected to the customer's premises <NUM> by a drop fibre <NUM> in which a plurality of distributed Bragg reflectors (i.e. Fibre Bragg Gratings (FBG)) <NUM> are embedded at regular intervals along the drop fibre, so that, when the fibre is cut to length, a grating <NUM> will be located within a specified maximum distance of the cut end <NUM>. To facilitate cutting the fibre at a suitable location between adjacent FBGs, for example within a preferred distance of a FBG whilst avoiding cutting into one of the FBGs, the location along the fibre of each of the plurality of FBGs is marked externally, e.g. on the exterior of the fibre or on the exterior of a cable comprising the fibre. This provides an efficient method of ensuring that a suitable optical reflector lies within a predetermined distance of the customer's end <NUM> of the drop fibre, even when the drop fibre is cut to length in-situ and the final length of the drop fibre is not known in advance. The maximum distance from the customer's end <NUM> of the drop fibre to the FBG <NUM> closest to the customer's end <NUM> of the drop fibre will be limited by the spacing. According to alternative embodiments, FBGs may be located periodically (i.e. at intervals) along the drop fibre. The intervals may be regular or irregular depending on, for example, ease of manufacture and, where the intervals are irregular, at intervals in which the maximum distance between adjacent FBGs falls within a predetermined value. This provides an efficient method of ensuring that a FBG will lie within a predetermined distance of the customer's end of the drop fibre, even when the drop fibre is cut to length in-situ and the final length of the drop fibre is not known in advance. A suitable spacing may be selected so that this distance will be much less than the total length of the drop fibre from the customer's end to the distribution point. The choice of spacing between FBGs will be a balance between cost and accuracy. Due to losses in the fibre itself, the lower the spacing, the higher the accuracy. According to an embodiment, a FBG spacing along the fibre of <NUM> is used as this will keep the round-trip loss to approximately <NUM> dB or less and so have a negligible effect on the measurement accuracy.

By keeping the distance along the fibre from the cut end to the closest FBG within limits by use of embedded gratings, the accuracy of loss measurements is increased while, as will be described later, the use of embedded optical reflectors reduces the number of steps required to check a field-fit connector.

FBGs <NUM> are passive components which can be fabricated by changing the refractive index of a fibre in multiple, periodic bands (schematically represented by the vertical black bars in the Figures). The pitch and width of these bands allows the FBG to be "tuned" to reflect a specific wavelength of light (the "FBG wavelength") and to reflect a certain proportion of the power at that wavelength. The width of the grating determines the bandwidth (also known as the full-width-half-maximum (FWHM)) of the reflector, centred on the central "Bragg wavelength". A bandwidth of <NUM> has been found to be acceptable, although other values may be preferred, depending on the circumstances applying in a particular network. This is a trade-off, as the narrower the grating bandwidth, the more precisely the test signal light source has to be controlled leading to higher costs, while a looser spec results in more spectrum being consumed. A variation in the test signal wavelength of ±<NUM> has been found to be acceptable, although higher precision may be desirable, depending on the circumstances applying in a particular network.

That is, the FBGs <NUM> send the optical power (or a proportion of the optical power) at a specific wavelength back towards the source, rather than allowing forward transmission. The FBGs <NUM> can be designed to reflect a specific wavelength (e.g. that does not interfere with data transmission). A FBG can be inscribed into a fibre very efficiently as the fibre is pulled from the preform in conventional optical fibre manufacture. According to an embodiment, the plurality of FBGs are configured to reflect light at the same wavelength. There is a need to avoid any wavelengths used in normal operation of the fibre. According to an embodiment, the preferred wavelength will correspond to the ITU reserved monitoring bands of <NUM>-<NUM>. In cases where <NUM> is used by engineers to test the network from the customer premises, <NUM> would be the preferred wavelength, although other wavelengths may be used on a particular network, depending on the circumstances applying to that particular network.

The reflectivity of the FBGs <NUM> is set during the manufacturing process and may be checked to ensure that the grating conforms to the required reflectivity. The fibre <NUM> is connected at the distribution point <NUM> (e.g. using a reliable fusion splicing technique) by direct termination onto the drop cable <NUM> or (as shown in the Figure) by a factory-fitted connector <NUM> (i.e. by a connector whose fitting, was carried out under optimum conditions at a manufacturing facility, and has been checked, e.g. by the manufacturer - either at the manufacturing facility or at a suitable test facility).

<FIG> shows a conventional field-fit connector <NUM> fitted to the end of the drop fibre <NUM> at the customer's premises <NUM>. The exact attachment method for a field-fit connector will be as specified by the manufacturer but will, in general, involve removal of outer coatings and cutting to length before insertion into and fixing to the connector <NUM>.

<FIG> shows test equipment <NUM> designed for connection to the field-fit connecter <NUM> on drop fibre <NUM> to measure the loss introduced by the fitting of field-fit connector <NUM> to the drop fibre <NUM>. The test equipment <NUM> comprises a light source <NUM> (Tx) and an optical receiver, e.g. a photodiode, <NUM> (Rx) connected to send and receive light, respectively through a factory fitted connector <NUM> that mates, in use, with the field-fit connector <NUM> of the drop fibre. The test equipment <NUM> also comprises power splitter or diplexer <NUM>. The diplexer <NUM> is a wavelength division multiplexer which can separate or combine specific wavelengths of light onto a single fibre. The diplexer <NUM> is configured to guide transmitted light <NUM> from the source <NUM> into the drop fibre via the diplexer <NUM> and the factory-fit connector <NUM> and to guide the reflected light <NUM> received via the factory-fit connector <NUM> and the diplexer <NUM> from the drop fibre to the receiver <NUM>. Factory-fit connector <NUM> presents an interface <NUM> to field-fit connector <NUM> for interchange of optical signals <NUM> and <NUM>. The power-level of the transmit light from the source may be measured or controlled. For example, the light source may be calibrated to transmit at a predetermined power level. This may include internal feedback control (not shown), for example by using a back-facet diode to monitor the optical power output. The reflected light received from the fibre is connected to an optical power meter (not shown) for measurement. Suitable optical power meters include the OPM1 Optical Power Meter from AFL of Duncan, SC <NUM>, USA. The embodiment may be implemented with both source <NUM> and receiver <NUM> configured to work with light in a narrow band around the FBG wavelength. That is, where the source <NUM> is configured to transmit at the FBG wavelength and source and receiver power levels are measured at the FBG wavelength.

The test equipment <NUM> may be constructed from standard optical components such as may be used to make a bidirectional optical transceiver. According to an embodiment, these components could consist of the diplexer, a receiver optical sub assembly (ROSA) and a transmitter optical sub assembly (TOSA). The ROSA could comprise an avalanche or PIN photodiode photodetector, trans-impedance amplifier and limiting amplifier. The TOSA could comprise a laser diode with back facet monitor and driver circuit. The whole may be controlled by a programmable microcontroller or equivalent electronic logic circuitry <NUM> configured to detect the difference between the power level of the light <NUM> sent by the source <NUM> and the power level of the light <NUM> received at the receiver <NUM>. A suitable display <NUM> may be used to provide the result to the field engineer.

<FIG> shows the test equipment <NUM> from <FIG> connected to the field-fit connecter <NUM> at the customer's premises. As shown in <FIG>, a test optical signal <NUM> from the source <NUM> passes through interface <NUM> of test equipment <NUM> and is connected into the customer's side of the field-fit connector <NUM>, from where it propagates along the drop fibre <NUM> to the FBG <NUM> closest to the field-fit connector <NUM> end of the fibre <NUM> (the "closest FBG"). The closest FBG <NUM> reflects <NUM> the test optical signal <NUM> (or at least a significant proportion of the test optical signal) back towards the field-fit connector <NUM> end of the fibre <NUM>. The light reflected at the closest FBG returns through the fibre <NUM> and passes through field-fit connector <NUM> and interface <NUM> of test equipment <NUM> to the receiver measuring device <NUM> shown in <FIG>. It will be noted that, unlike conventional methods, there is no requirement for the drop fibre to be lit from the exchange.

A small component (typically no more than <NUM>% for FBGs), of the test optical signal will be passed by the closest FBG and will reach the next-closest (or "second") FBG. A sub-component of the test optical signal, reflected at the second FBG will return to the closest FBG and will experience a large degree (typically at least <NUM>% for FBGs) of reflection, with only a very small sub-sub-component (i.e. no more than <NUM>%) of the test optical signal passing the closest FBG again and arriving at the test equipment. Reflected sub-components of the test optical signal from any third or subsequent FBGs will be correspondingly smaller. While all references to the power levels of reflections from the closest FBG are understood to include reflected sub-components from any second, third or subsequent FBGs, the effect of such reflected sub-components on the power readings will be so small as not to materially affect the outcome.

Use of test equipment <NUM> allows measurement of the power level of the light that is introduced into the drop fibre <NUM> through the field-fit connector <NUM> and the power level of the light reflected back from the drop fibre FBG <NUM> through the field-fit connector <NUM>. The reflectivity of the FBG is also known and we can therefore calculate the loss introduced by the field-fit connector, as follows: <MAT>.

For example, where the power level of the source transmitted light (tx_pwr) is -<NUM> dBm, the reflectivity of the grating is -<NUM> dB and the power level of the received light (Rx_pwr) is -<NUM> dBm, then the loss is given by: <MAT>.

The embodiment may be implemented with an additional operation involving a calibration plug, as shown in <FIG> shows a calibration plug <NUM> which comprises an optical reflector (e.g. a grating) which has the same reflectivity at the FBG wavelength as the FBGs inserted into the fibre. Use of the calibration plug <NUM> allows the transmit power to be accurately measured at the interface <NUM> of test equipment <NUM> taking into account loss introduced by factory-fit connector <NUM>. The calibration plug can be plugged into the test equipment interface <NUM> in place of the field-fit connector and the power from the transmitted light that is reflected back to the test equipment by the calibration plug can then be measured by the test equipment in the normal way. This measurement provides a reference against which measurements carried out on field-fit connectors may be compared. Given the reflectivity of the calibration plug optical reflector is known, the power level of the transmitted light provided at test equipment interface <NUM> to the field-fit connector <NUM> by the test equipment <NUM> may be accurately calculated and losses inherent in the test equipment eliminated from the calculation of loss associated with the field-fit connector.

Although of particular benefit in testing field-fit connectors, the embodiments have application in testing the performance of other types of optical fibre connector.

The invention relates to an optical fibre comprising a plurality of embedded optical reflectors distributed periodically along the length of the fibre. The invention also relates to a method of quantifying loss associated with an optical connector that is connected to optical fibre comprising a plurality of embedded optical reflectors distributed periodically along the length of the fibre. The method comprises: inserting an optical signal into the fibre through the optical connector; measuring a component of the optical signal reflected by at least one of the plurality of embedded optical reflectors, in which the component is received through the optical connector; calculating the difference in power level between the inserted and reflected signals; and quantifying, based on the calculated power level difference and the reflectivity of the embedded optical reflector, the loss associated with the optical connector.

Claim 1:
A method of quantifying loss associated with an optical connector (<NUM>) that is connected to an end of an optical fibre (<NUM>);
in which the optical fibre (<NUM>) comprises a plurality of embedded optical reflectors (<NUM>) distributed periodically along the length of the fibre (<NUM>);
in which the method comprises:
inserting an optical signal into the fibre (<NUM>) through the optical connector (<NUM>);
measuring a component of the optical signal reflected by at least one of the plurality of embedded optical reflectors (<NUM>), in which the component is received through the optical connector (<NUM>);
calculating the difference in power level between the inserted and reflected signals; and
quantifying, based on the calculated power level difference and the reflectivity of the embedded optical reflectors (<NUM>), the loss associated with the optical connector (<NUM>).