Optical transceiver having an otdr mode, and a method of obtaining test data for testing an optical fiber

TECHNICAL FIELD

p-0002The invention relates generally to the field of optical transceivers.

BACKGROUND

p-0003It is known to use an optical time-domain reflectometer (OTDR) in the form of a standalone optoelectronic test instrument to characterize an optical fiber. The

p-0004OTDR injects a series of optical pulses into an optical fiber under test, and detects light that is Rayleigh scattered and reflected back from locations in the fiber where the fibers index of refraction changes. The return pulses are, for example, measured and integrated as a function of time, and can be plotted as a function of fiber length. Such an OTDR may be used for estimating a fiber's length and overall attenuation, including indicating splice and connection losses, for example. The OTDR can also be used to locate faults, such as breaks, in the fiber. It is also known to provide small form factor fiber-optic transceiver modules having built-in test capability.

SUMMARY

p-0005In accordance with the invention, there is provided an optical transceiver as claimed in claim <b>1</b>.

p-0006In accordance with a further aspect of the invention, there is provided a method of obtaining test data for an optical fiber as claimed in claim <b>9</b>.

BRIEF DESCRIPTION OF THE DRAWINGS

p-0007In order that the invention may be well understood, various embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings, in which:

p-0008<figref idrefs="DRAWINGS">FIG. 1</figref> is a schematic plan view illustrating selected features of an optical transceiver in a communications mode;

p-0009<figref idrefs="DRAWINGS">FIG. 2</figref> is a schematic plan view illustrating selected features of the optical transceiver in an optical time domain reflectometer (OTDR) mode;

p-0010<figref idrefs="DRAWINGS">FIG. 3</figref> is a plan view in partial cross-section showing further details of the construction of one embodiment of the optical transceiver;

p-0011<figref idrefs="DRAWINGS">FIG. 4</figref> is an end view taken on section IV-IV in <figref idrefs="DRAWINGS">FIG. 3</figref>;

p-0012<figref idrefs="DRAWINGS">FIG. 5</figref> is a plan view showing a detail of a portion of a guide arrangement shown in <figref idrefs="DRAWINGS">FIG. 3</figref>;

p-0013<figref idrefs="DRAWINGS">FIG. 6</figref> is a functional diagram illustrating electronic circuitry of the optical transceiver;

p-0014<figref idrefs="DRAWINGS">FIG. 7</figref> is a schematic plan view illustrating an alternative embodiment of the optical transceiver;

p-0015<figref idrefs="DRAWINGS">FIG. 8</figref> is a plan view illustrating an optical communications subsystem comprising the optical transceiver;

p-0016<figref idrefs="DRAWINGS">FIG. 9</figref> is a flow diagram illustrating a method of obtaining test data for an optical fiber; and

p-0017<figref idrefs="DRAWINGS">FIG. 10</figref> is a further flow diagram illustrating a method of obtaining test data for an optical fibre.

p-0018Drawings are schematic and not to scale.

DETAILED DESCRIPTION

p-0019<figref idrefs="DRAWINGS">FIGS. 1 to 3</figref> illustrate selected aspects of an optical transceiver <b>100</b> having a body <b>110</b> supporting a transmitter arrangement indicated for convenience by the bounds of broken line <b>120</b>, a receiver arrangement indicated for convenience by the bounds of broken line <b>140</b>, and electronic circuitry <b>600</b> operable to control the transmission and receiver arrangements <b>120</b>, <b>140</b>. The electronic circuitry <b>600</b> includes electrically conductive contacts <b>601</b> for connecting with a compatible circuit (not shown) of a host device (not shown), to enable the host device, in a communications mode of the transceiver <b>100</b>, to use electrical signals to cause optical communications signals to be transmitted and received by the transmission and receiver arrangements <b>120</b>, <b>140</b>. The transceiver <b>100</b> also has an OTDR mode in which the electronic circuitry <b>600</b> causes the transmitter arrangement <b>120</b> to transmit an OTDR signal, the back-scattered radiation from which is received by the receiver arrangement <b>140</b>.

p-0020The transmitter arrangement <b>120</b> includes a transmitter channel <b>121</b> having a generally planar end face <b>122</b> and an axially opposite generally planar end face <b>123</b> for mating with a generally planar end face of an external optical fiber <b>124</b>. The transmitter channel <b>121</b> comprises, for example, silicon dioxide (silica) or other material suitable for allowing propagation of optical signals, and has a generally circular cross-section surrounded by cladding (<b>330</b>, <figref idrefs="DRAWINGS">FIG. 3</figref>) to facilitate total internal reflection. The transmitter channel <b>121</b> also includes a light source and optics arrangement <b>125</b> for generating optical signals and injecting the optical signals into the planar end face <b>122</b> of the transmitter channel <b>121</b>. The light source conveniently comprises a laser, for example a semiconductor laser diode (<b>325</b>, <figref idrefs="DRAWINGS">FIG. 3</figref>) such as a vertical cavity surface emitting laser (VCSEL), and the optics conveniently comprises one or more lenses (<b>326</b>, <figref idrefs="DRAWINGS">FIG. 3</figref>).

p-0021The receiver arrangement <b>140</b> includes a receiver channel <b>141</b> having a generally planar end face <b>143</b> for mating with a generally planar end face of an external optical fiber <b>144</b>, and an axially opposite generally planar end face <b>142</b>. The receiver channel <b>141</b> can also comprise, for example, silicon dioxide or other material suitable for allowing propagation of optical signals, and has a generally circular cross-section surrounded by cladding (<b>350</b>, <figref idrefs="DRAWINGS">FIG. 3</figref>) to facilitate total internal reflection. The receiver arrangement <b>140</b> also includes a photodetector and optics arrangement <b>145</b> for receiving optical signals from the receiver channel <b>141</b> and converting the optical signals to electrical signals. The photodetector (<b>345</b>, <figref idrefs="DRAWINGS">FIG. 3</figref>) conveniently comprises an avalanche diode configured to operate close to avalanche mode, although for some embodiments alternative photodetectors may be appropriate, for example a p-i-n diode photodetector. The receiver optics conveniently comprises one or more lenses (<b>346</b>, <figref idrefs="DRAWINGS">FIG. 3</figref>). The laser diode <b>325</b> and photodetector <b>345</b> are operable to respectively generate and detect optical signals having mutually similar wavelength, for example in the region of <b>850</b> nm or any other suitable wavelength.

p-0022The transceiver channel <b>121</b> and the receiver channel <b>141</b> include respective beam splitters <b>127</b>, <b>147</b>. The transceiver <b>100</b> further includes a lateral guide <b>160</b> extending laterally of the respective longitudinal axes of the transmitter and receiver channels <b>121</b>, <b>141</b> adjacent the longitudinal locations of the beam splitters <b>127</b>, <b>147</b>. The lateral guide <b>160</b> includes a lateral guide channel <b>161</b> (<figref idrefs="DRAWINGS">FIG. 3</figref>) having a generally circular cross-section and comprising, for example, silicon dioxide. The lateral guide channel <b>161</b> is provided with radially outer cladding <b>162</b> to facilitate propagation of guided optical signals along the guide channel <b>161</b>. The lateral guide <b>160</b> also conveniently includes, at opposite longitudinal end portions thereof, respective plano-convex lenses <b>163</b>, <b>164</b> for guiding optical signals between the transmission and receiver channels <b>121</b>, <b>141</b> and the lateral guide channel <b>161</b>.

p-0023The transceiver <b>100</b> is also provided with a shutter arrangement including an optical shutter <b>170</b> having an electrical connection to the circuitry <b>600</b>. The optical shutter <b>170</b> is arranged laterally across the longitudinal axis of the lateral guide channel <b>161</b>. The optical shutter <b>170</b> is controllable through the electrical connection <b>171</b> by the circuitry <b>600</b> to assume at least two conditions. In one, open, condition of the optical shutter <b>170</b>, propagation of optical signals along the lateral guide channel <b>161</b> is permitted, at least for wavelengths in the region of the transmission wavelength. In an alternative, shuttered, condition of the optical shutter <b>170</b>, propagation of optical signals along the lateral guide channel <b>161</b> is significantly reduced or prevented, at least for wavelengths in the region of the transmission wavelength. The optical shutter <b>170</b> can take the form of an optical modulator, for example a liquid-crystal light valve, or any other appropriate shutter device with sufficiently fast switching capability.

p-0024The beam splitter <b>127</b> of the transmitter channel <b>121</b> includes mutually oppositely facing substantially planar adjacent faces extending laterally across the transmitter channel <b>121</b>, and is disposed in a vertical plane at a precisely determined angle with respect to the longitudinal centre axis A-A of the transmitter channel <b>121</b>. The angle a (<figref idrefs="DRAWINGS">FIG. 3</figref>), is defined as the smallest angle. measured in a horizontal plane, between the vertical plane of the beam splitter and a vertical plane including the centre axis A-A of the transmitter channel <b>121</b>. A suitable material may be disposed between the adjacent faces, for example as a coating on at least one of the faces, to obtain desired transmission characteristics through the beam splitter <b>127</b>. For example, the coating, if present, could determine what proportion of a signal passes along the transmitter channel <b>121</b>, and what proportion is reflected.

p-0025The beam splitter <b>147</b> of the receiver channel <b>141</b> includes mutually oppositely facing substantially planar adjacent faces extending laterally across the receiver channel <b>141</b>, and is disposed in a vertical plane at a precisely determined angle with respect to the longitudinal centre axis B-B of the transmitter channel <b>121</b>. The angle b (<figref idrefs="DRAWINGS">FIG. 3</figref>), is defined as the smallest angle, measured in a horizontal plane, between the vertical plane of the beam splitter <b>147</b> and a vertical plane including the centre axis B-B of the receiver channel <b>141</b>. A suitable material may be disposed between the adjacent faces, for example as a coating on at least one of the faces, to obtain desired transmission characteristics through the beam splitter <b>147</b>. For example, the coating, if present, could determine what proportion of a signal passes through the beam splitter <b>147</b>, and what proportion is reflected.

p-0026As illustrated in <figref idrefs="DRAWINGS">FIG. 1</figref>, in the communications mode, the transceiver <b>100</b> is operable to transmit outbound signals along a transmission path shown by unbroken line <b>126</b> through the transmitter channel <b>121</b> into the external optical fiber <b>124</b>, and to receive incoming signals along a received signal path <b>146</b>. The beam splitter <b>127</b> in the transmitter channel <b>121</b> permits transmission therethrough along the path <b>126</b> of a proportion of the energy of an outbound signal. Some proportion of the outbound signal will be reflected from the path <b>126</b> by the beam splitter <b>127</b> and dispersed. The beam splitter <b>147</b> is configured to permit passage therethrough along the received signal path <b>146</b> of some proportion of the energy of an incoming signal. Some proportion of the incoming signal will be reflected from the path <b>146</b> by the beam splitter <b>147</b> and dispersed.

p-0027Outbound signals along the path <b>126</b> cause back-scattered radiation to be reflected along a return path, indicated generally by broken line <b>128</b>, in an opposite direction to the outbound signal. The angle a of the beam splitter <b>127</b> is configured to reflect a proportion of the back-scattered signal laterally of the longitudinal centre axis A-A through the guide channel <b>161</b>, towards the beam splitter <b>147</b> in the receiver channel <b>141</b>. In the communications mode, the optical shutter <b>170</b> is in the shuttered condition and reduces or prevents passage of signals laterally through the guide channel <b>161</b> into the receiver channel <b>141</b>.

p-0028As illustrated in <figref idrefs="DRAWINGS">FIG. 2</figref>, in the OTDR mode, the transceiver <b>100</b> is operable to transmit an outbound OTDR signal along the path <b>126</b>, for example in the form of a predetermined optical pulse. The resultant back-scatter of the reflected OTDR signal is reflected along the return path <b>128</b>, and a proportion of this back-scatter is further reflected by the beam splitter <b>127</b> into the lateral guide channel <b>161</b>. In the OTDR mode, the optical shutter <b>170</b> is in a condition to permit propagation of the resultant back-scatter of the reflected OTDR signal through the lateral guide channel <b>161</b>, along a path indicated generally by the reference <b>163</b>, to the beam splitter <b>147</b> in the receiver channel <b>141</b>. The angle b of the beam splitter <b>147</b> is configured to reflect a proportion of the back-scatter received from the lateral guide channel <b>161</b> along path <b>148</b> to the receiver optics and photodetector <b>145</b>. In <figref idrefs="DRAWINGS">FIGS. 1 to 3</figref>, the beam splitter <b>127</b> is configured to reflect back-scattered signals laterally at about 90° to the longitudinal centre axis A-A. However, in alternative embodiments, any convenient alternative angle can be selected, and the beam splitter <b>147</b> and lateral guide <b>160</b> reconfigured accordingly. From the above description, it will be apparent that a guide arrangement is provided for guiding, in the OTDR mode, a reflected OTDR signal along a path from the transmission channel into the receiver channel. To facilitate detection of back-scatter from the reflected OTDR signal, the electronic circuitry <b>600</b> causes transmission of communication signals through the receiver channel <b>141</b> along the path <b>146</b> to cease in OTDR mode, as described in further detail below.

p-0029One exemplary construction and arrangement of the lateral guide <b>160</b> and the transceiver and receiver channels <b>121</b>, <b>141</b> will now be described mainly with reference to <figref idrefs="DRAWINGS">FIGS. 3 to 5</figref>. <figref idrefs="DRAWINGS">FIG. 3</figref> is a plan view of portions of the optical transceiver <b>100</b> in partial cross-section through a horizontal plane including the longitudinal centre axes A-A and B-B of the transmission and receiver channels <b>121</b>, <b>141</b>. <figref idrefs="DRAWINGS">FIG. 3</figref> shows the ends of two external optical fibers <b>124</b>, <b>144</b> with cladding <b>360</b>, <b>370</b> protruding from respective ferrules <b>361</b>, <b>371</b> of a cable end connector, for example an LC connector or any other appropriate type of connector. The protruding ends are receivable in recesses of the transceiver <b>100</b>, such that respective end faces of the external optical fibers <b>124</b>, <b>144</b> are biased into face-to-face abutment with the respective generally planar end faces <b>123</b>, <b>143</b> of the transmission channel <b>121</b> and the receiver channel <b>141</b>. The transmitter channel <b>121</b> and the receiver channel <b>141</b> respectively form part of a transmitter subassembly <b>301</b> and a receiver subassembly <b>310</b>. The transmitter light source and optics <b>325</b>, <b>326</b> and the receiver photodetector and optics <b>345</b>, <b>346</b> respectively form part of the transmitter subassembly <b>301</b> and the receiver subassembly <b>310</b>, and are mounted in respective alignment with the transmitter and receiver channels <b>121</b>, <b>141</b>. Components of the transmitter and receiver subassemblies <b>301</b>, <b>310</b> and the lateral guide <b>160</b> are fixedly mounted in alignment relative to one another, for example by fixed mounting to a common rigid support <b>302</b>.

p-0030In one exemplary embodiment, a short length of an optical fiber with cladding, such as 50 μm/125 μm 0M3 optical fiber, is encased at least partially around the radially outer periphery of the cladding with a plastics material to form a body <b>303</b>, <b>313</b>. At least one of the base and sides of the plastics body is flat and manufactured to predetermined tolerances, to facilitate alignment relative to the common support <b>302</b> and/or another subassembly <b>301</b>, <b>310</b>. A cut <b>305</b>, <b>315</b> is made at a desired angle a, b to the optical fiber's longitudinal axis. vertically through the plastics body and the optical fiber. The two resultant halves are mated together in mutual alignment, for example by adhering opposite portions of the plastics together. The resulting mating internal end faces of the optical fiber provide a beam splitter <b>127</b>, <b>147</b>.

p-0031Subsequently, a vertical incision <b>306</b> , <b>316</b> is made laterally partially through the plastics body <b>303</b>, <b>313</b>, cladding <b>330</b>, <b>350</b>, and optical fiber <b>121</b>, <b>141</b> in the region of the beam splitter <b>127</b>, <b>147</b> using a circular rotating cutting tool having a cutting edge with a curved edge profile. The vertical incision <b>306</b>, <b>316</b> provides a passage into which an end of the lateral guide <b>160</b> can pass, and exposes the optical fiber core so that a plano-convex lens <b>163</b>, <b>164</b> of the lateral guide <b>160</b> can form an optical connection with the transmission or receiver channel <b>121</b>, <b>141</b>. The incision <b>306</b>, <b>316</b> is made at a precise longitudinal location of the optical fiber relative to the beam splitter <b>127</b>, <b>147</b> such that, when the lateral guide <b>160</b> is inserted in the incision <b>306</b>, <b>316</b>, the longitudinal centre axis of the lateral guide channel <b>161</b> is generally aligned with the centre of the beam splitter <b>127</b>, <b>147</b>, as best seen in <figref idrefs="DRAWINGS">FIG. 5</figref>.

p-0032Various functions and operations of the electronic circuitry <b>600</b> will now be described with reference to <figref idrefs="DRAWINGS">FIG. 6</figref>. In the communications mode, the electronic circuitry <b>600</b> receives host device electrical control signals which are processed through a transmitter electrical signal processing channel including an equalizer <b>610</b> and a light source driver <b>611</b>, for example a laser driver, to cause the laser <b>325</b> to transmit optical signals through the transmission channel <b>121</b>. The electronic circuitry <b>600</b> also receives electrical signals from the photodetector <b>345</b> which, in the communications mode, are processed through a receiver electrical signal processing channel including a transimpedance amplifier (TIA) <b>620</b> and a limiter <b>621</b> for output to the host device. The electronic circuitry <b>600</b> comprises control logic <b>630</b> including OTDR control logic <b>631</b>. The control logic <b>630</b> can communicate with control logic <b>630</b> of another transceiver <b>100</b> at an opposite end of an external optical fiber connected to the transmitter channel <b>121</b> using a modulator <b>613</b> and demodulator <b>623</b>. The modulator <b>613</b> is controlled by modulation control logic <b>632</b> to provide transmitter channel sideband communications, modulated over a much lower frequency than the primary communications signal frequency. A demodulator <b>623</b> is controlled by the modulation control logic <b>632</b> to receive modulated sideband communications from the other transceiver <b>100</b>.

p-0033Conveniently, the transceiver <b>100</b> is an SFP-type transceiver, that is, is in conformity with the requirements set out by the SFF committee in various documents for SFP, SFP+, SFF or similar transceivers (example SFF specification documents: INF-8074i, SFF-8431). In alternative embodiments, any other suitable type of optical transceiver can be employed, for example other transceivers with relatively small form factors such as XFP type transceivers and others. In some embodiments, the transceiver <b>100</b> is configured to be removably hot pluggable into connection with a host device circuit. The electronic circuitry <b>600</b> may be provided in any convenient form. For example, all or parts of the circuitry <b>600</b> can comprise hardware logic in the form of an integrated circuit and/or programmable logic such as a field programmable gate array. The circuitry <b>600</b> may comprise a printed circuit board interconnecting various discrete components. The circuitry <b>600</b> may comprise an embedded computer system with program instructions stored in memory.

p-0034The control logic <b>630</b> can communicate with a host device ASIC in any suitable manner, for example through a suitable communications interface <b>640</b>, such as I2C or other suitable interface. For example, the control logic <b>630</b> can store data in predetermined locations in a memory <b>641</b>, for example EEPROM or other suitable memory, and the host device can access the data by polling the memory <b>641</b> over the interface <b>640</b>. In the exemplary embodiment of the SFP-type transceiver <b>100</b>, the SFP-type diagnostic and monitoring interface can be used for communication with the host device, by using available fields to extend the protocol.

p-0035After entering the OTDR mode of the transceiver <b>100</b>, the OTDR control logic <b>631</b> operates a switch <b>612</b> to prevent the normal flow of the host device electrical control signals through the transmitter electrical signal processing channel to the laser <b>325</b>, and operates a switch <b>622</b> to prevent the normal flow of electrical signals through the receiver electrical signal processing channel to the host device. Conveniently, the OTDR control logic <b>631</b> communicates with the host device to coordinate control of the switches <b>612</b>, <b>622</b>.

p-0036The OTDR control logic <b>631</b> issues signals to the optical shutter <b>170</b> through the connection <b>171</b> to cause the shutter to open to permit propagation of back-scatter through the lateral guide channel <b>161</b>. The OTDR control logic <b>631</b> controls the light source driver <b>611</b> to generate an OTDR test signal, for example an OTDR pulse, that travels into the transmission channel <b>121</b> and the corresponding connected external optical fiber <b>124</b>. The OTDR test signal generates a reflected OTDR signal, in the form of back-scatter generated by reflection of the test signal by the material of the optical fiber <b>124</b> as the test signal travels along the optical fiber <b>124</b>. The reflected OTDR signal travels back along the transmission channel <b>121</b> in the direction of the path <b>128</b> towards the beam splitter <b>127</b>. The reflected OTDR signal is guided along a path <b>128</b>, <b>163</b>, <b>148</b> from the transmission channel <b>121</b> into the receiver channel <b>141</b> to the photodetector and optics <b>145</b> by the guide arrangement comprising the beam splitter <b>127</b>, the lateral guide channel <b>161</b> and the beam splitter <b>147</b>. Part of the energy of the reflected OTDR signal is redirected from the path at the beam splitters <b>127</b>, <b>147</b>, and dispersed. The portion of the reflected OTDR signal guided to the photodetector <b>345</b> is detected over a predetermined time period relative to generation of the OTDR pulse, and is converted to electrical signals which are processed by the TIA <b>620</b>. The OTDR logic <b>631</b> samples the signals and stores the resulting OTDR data in memory. A dedicated OTDR memory <b>642</b>, for example nonvolatile memory such as RAM, or any other suitable memory type, may be provided for storing the OTDR data.

p-0037The stored OTDR data can be processed and analysed in accordance with known OTDR processes. For example, the detected OTDR pulse response can be processed to identify anomalies in the external fiber-optic cable, and the approximate distance of such anomalies from the transceiver <b>100</b>. The OTDR control logic <b>631</b> may include OTDR data processing logic for performing some analysis of the OTDR data. Alternatively, the OTDR data can be communicated by the OTDR control logic <b>631</b> to the host device using, for example, the I2C interface <b>640</b>, and some or all of the processing and analysis of the OTDR data can be performed off-transceiver.

p-0038<figref idrefs="DRAWINGS">FIG. 7</figref> shows an alternative embodiment of the transceiver <b>100</b>, in OTDR mode, in which further beam splitters <b>727</b>, <b>747</b> are respectively provided in the transmitter and receiver channels <b>121</b>, <b>141</b>. The further beam splitter <b>727</b> in the transmitter channel <b>121</b> is angled oppositely to the beam splitter <b>127</b>, so as to direct the OTDR test signal laterally of the longitudinal axis A-A of the transmitter channel <b>121</b> towards the further beam splitter <b>747</b> in the receiver Channel <b>141</b>. A lateral light guide <b>760</b> is provided to guide the test signal from the further beam splitter <b>727</b> to the further beam splitter <b>747</b> along a lateral path indicated by the dotted line <b>763</b>. The OTDR test signal is further reflected by the beam splitter <b>747</b> into the receiver channel <b>141</b> and towards the photodetector and optics <b>145</b>. Some proportion of the energy of the OTDR test signal is reflected away from the path <b>763</b>, <b>748</b> at the beam splitters <b>727</b>, <b>747</b>. A further shutter arrangement <b>770</b> is arranged laterally of the longitudinal axis of the lateral guide <b>760</b>. The beam splitters <b>727</b>, <b>747</b>, the further lateral guide <b>760</b> and the further shutter arrangement <b>770</b> can be formed in corresponding manner to the previously described beam splitters <b>127</b>, <b>147</b>, lateral guide <b>160</b> and shutter arrangement <b>170</b>. Using the embodiment of <figref idrefs="DRAWINGS">FIG. 7</figref>, the OTDR control logic <b>631</b> can conveniently time and record detection of the reflected OTDR signals relative to the original OTDR test signal.

p-0039In operation of the transceiver <b>100</b> described above with reference to <figref idrefs="DRAWINGS">FIGS. 1 to 7</figref>, to facilitate detection of back-scatter from the reflected OTDR signal, the electronic circuitry <b>600</b> causes transmission of communication signals through the receiver channel <b>141</b> along the path <b>146</b> to cease in OTDR mode. Additionally or alternatively, the control logic may operate to determine that transmissions from an opposite end of an optical fiber received in the receiver channel <b>141</b> have failed. In response to such a determination, for example, the control logic may enter OTDR mode without any requirement to cause the cessation of communication signals through the receiver channel <b>141</b> along the path <b>146</b>.

p-0040<figref idrefs="DRAWINGS">FIG. 8</figref> is a plan view illustrating an optical data communications subsystem <b>800</b> comprising a first optical transceiver <b>100</b><i>b </i>having a first transmitter arrangement <b>120</b><i>b </i>and a first receiver arrangement <b>140</b><i>b </i>connected by respective optical fibers <b>124</b>, <b>144</b> to a second receiver arrangement <b>140</b><i>a </i>and a second transmitter arrangement <b>120</b><i>a </i>of a second transceiver <b>100</b><i>a</i>. Components of the subsystem shown in <figref idrefs="DRAWINGS">FIG. 8</figref> having a reference including an appended letter can, for example, correspond to components having a like reference without the appended letter described above with reference to <figref idrefs="DRAWINGS">FIGS. 1 to 7</figref>.

p-0041The OTDR control logic <b>631</b> may be configured, for example, to place the transceiver <b>100</b> in OTDR mode and obtain and store the OTDR test data according to a predetermined schedule and/or in response to communications received from the host device, for example over the I2C interface <b>640</b>. In one method of obtaining OTDR test data for an optical fiber <b>124</b> in the optical data communications subsystem <b>800</b>, the OTDR control logic of the circuitry <b>600</b><i>a</i>, with the transceiver <b>100</b><i>a </i>in the communications mode, uses modulated sideband communications to request the OTDR control logic of the circuitry <b>600</b><i>b </i>to temporarily interrupt operational signal transmission through the first transmitter arrangement <b>120</b><i>b </i>(step <b>901</b>, <figref idrefs="DRAWINGS">FIG. 9</figref>). The OTDR control logic of the circuitry <b>600</b><i>a </i>then places the transceiver <b>100</b><i>a </i>into OTDR mode. The optical shutter <b>170</b><i>a </i>is caused to permit propagation of signals therethrough, and an OTDR test signal is transmitted using the second transmitter arrangement <b>120</b><i>a </i>to generate back-scatter in the optical fiber <b>124</b> to be tested (step <b>902</b>, <figref idrefs="DRAWINGS">FIG. 9</figref>). The back-scatter from the optical fiber <b>124</b> is guided along the path <b>128</b>, <b>163</b>, <b>148</b> by the beam splitters <b>127</b><i>a, </i><b>147</b><i>a </i>and received at the detector <b>145</b><i>b </i>of the second receiver arrangement <b>140</b><i>a </i>(step <b>903</b>, <figref idrefs="DRAWINGS">FIG. 9</figref>).

p-0042The OTDR control logic of the circuitry <b>600</b><i>a </i>stores the resultant OTDR data, causes the optical shutter <b>170</b><i>a </i>to close, and returns the transceiver <b>100</b><i>a </i>to the communications mode. The OTDR control logic of the circuitry <b>600</b><i>a </i>may then, using modulated sideband communications, instruct the OTDR control logic of the circuitry <b>600</b><i>b </i>to recommence operational signal transmission in the communications mode. Alternatively, such an instruction may not be required, for example where the first transceiver <b>100</b><i>b </i>is instructed to temporarily interrupt operational signal transmission for a predetermined time period. Causing the transceiver <b>100</b><i>b </i>coupled to the opposite end of the external optical fiber <b>124</b>, to stop transmission of communications signals to the second receiver arrangement <b>140</b><i>a </i>facilitates accurate detection of the back-scatter from the OTDR test signal using the photodetector <b>145</b><i>a </i>of the second receiver arrangement <b>140</b><i>a. </i>

p-0043Following detection of a failure in normal operational signalling in the communications mode, or of a failure in modulated sideband communications, prior to performing OTDR testing on the transceiver <b>100</b><i>a, </i>the OTDR control logic of the circuitry <b>600</b><i>a </i>may communicate with the OTDR control logic of the circuitry <b>600</b><i>b </i>through a network connected to the host devices respectively hosting the first and second transceivers <b>100</b><i>b, </i><b>100</b><i>a </i>to interrupt the first transmitter arrangement <b>120</b><i>b. </i>Such communications between the transceiver <b>100</b> and host device may, for example, be performed using the I2C diagnostic interface of the second transceiver <b>100</b><i>a, </i>or in any other convenient manner. Exemplary host devices may take the form of a host bus adapter or converged network adapter with firmware supporting reading of OTDR data and extended data communications from the transceiver device <b>100</b>, for example using the I2C interface <b>640</b>, or a switch or storage controller.

p-0044<figref idrefs="DRAWINGS">FIG. 10</figref> illustrates a method of operation embodied in the OTDR control logic <b>631</b>, with respect to the optical data communications subsystem <b>800</b>. Following initiation of the method (step <b>1000</b>), the control logic <b>631</b> of transceiver <b>100</b><i>a </i>determines (step <b>1001</b>) whether the OTDR mode condition has been established, for example in accordance with a predetermined periodic schedule or in response to a communication received through the communications interface <b>640</b>. The OTDR mode condition can be established, for example by setting a bit in a predetermined location in memory, for example in EEPROM <b>641</b>, to a predetermined state. If the OTDR mode condition has not been established, the OTDR control logic <b>631</b> determines in step <b>1002</b> whether the receiver link comprising external optical fiber <b>144</b> and the first transmitter arrangement <b>120</b><i>b </i>has failed, for example because no signal is presently detected at the photodetector <b>145</b><i>a. </i>If the receiver link <b>144</b>, <b>120</b><i>b </i>has not failed, the control logic <b>631</b> returns to the start (step <b>1000</b>) to re-initiate the routine. If a failure of the receiver link <b>144</b>, <b>120</b><i>b </i>is determined, the control logic <b>631</b> proceeds to determine (step <b>1003</b>) whether an auto-OTDR condition is set, for example by setting of a bit in a predetermined location in a memory of the circuitry <b>600</b> to a predetermined state. If the auto-OTDR condition is not set, the OTDR control logic <b>631</b> returns to the start <b>1000</b>.

p-0045If the OTDR control logic <b>631</b> determines that the OTDR mode condition of the transceiver <b>100</b><i>a </i>is established, or that the auto-OTDR condition is set and the receiver link <b>144</b>, <b>120</b><i>b </i>has failed, the OTDR control logic <b>631</b> proceeds to configure (step <b>1004</b>) the receiver and transmitter arrangements <b>120</b><i>a, </i><b>140</b><i>a </i>for OTDR operation, including setting an OTDR cycle count value to a predetermined start value N, using a predetermined location in a memory of the circuitry <b>600</b>. In general, the predetermined count start value N is greater than <b>1</b>, so that the results of multiple OTDR pulse cycles can be stored and averaged. In step <b>1005</b> the OTDR control logic <b>631</b> causes the second transmitter arrangement <b>120</b><i>a </i>to launch an OTDR laser pulse in the external fiber <b>124</b> to be tested. In step <b>1006</b> the optical modulator <b>170</b><i>a </i>is unshuttered, and in step <b>1007</b> the back-scatter from the OTDR test pulse is received by the photodetector <b>145</b><i>a </i>and stored by the OTDR control logic <b>631</b> in the OTDR memory <b>642</b>. In step <b>1008</b>, the stored test values are averaged, for example by calculating a walking average. The averaging can be performed, for example, by the controller logic <b>630</b> of the transceiver <b>100</b><i>a, </i>or can alternatively be performed by another device to which the stored OTDR test values have been communicated.

p-0046In step <b>1009</b>, the OTDR control logic <b>631</b> determines whether the OTDR count has reached a predetermined stop value, for example whether the OTDR count has been reduced to 0. If the predetermined stop value has been reached, the OTDR control logic returns to the start condition <b>1000</b>. If the predetermined stop value has not been reached, the OTDR control logic <b>631</b> adjusts the stored count value, for example by reducing the stored count value by <b>1</b> (step <b>1010</b>) and returns to step <b>1004</b> to perform a further OTDR test cycle. It will be apparent that the sequential order of at least some of the steps above can be varied if desired. By averaging the OTDR test results over many OTDR cycles, the accuracy of OTDR readings can be increased.

p-0047At least some embodiments of the invention facilitate the provision of a relatively compact and relatively cheap optical transceiver that can automatically and remotely obtain OTDR data for characterising an optical subsystem, including for identifying the presence and/or location of fiber breaks. and/or that can be provided in conformity with certain optical transceiver standards, for example SFP-type specifications. At least some embodiments facilitate the avoidance of inconvenience and/or lengthy procedures for placing an optical line in shutdown mode prior to OTDR testing. Utilisation of photodetector capability already present in a transceiver without OTDR capability facilitates compactness and/or reduced cost embodiments.