Abstract: A laser for generating laser light pulses comprises a cavity containing an active optical gain medium (102) and a spectral filtering device (104), a delay device (110) for delaying light by a predetermined delay time (?t), means (106) for extracting a portion of laser light from the cavity, launching said portion into said delay means (110) and returning the delayed portion to the optical gain medium (102), control means (116) operable to activate the gain medium for a first time period (C1) to produce a first laser light pulse (LP1) having a duration that is less than the delay time (?t), and activate the gain medium for a second time period (C2) while a said delayed portion of the first light pulse that has been delayed by the delay means (UO) is traversing the gain medium (102), thereby to produce a second laser pulse (LP2) having a shorter duration and faster risetime than the first laser light pulse (LP1), and output means (108) for outputting the second laser light pulse (LP2).

Abstract: A DMT (Discrete Multi-Tone) test method is employed by a single test device to estimate the theoretical and practical data rates of a cable under test for a pair of target ADSL (Asymmetric Digital Subscriber Line) DMT modems. The DMT test can also be used to analyze the cause of a problem, or to predict a problem, whereby a pair of ADSL DMT modems could fail to synchronize. The method includes (a) measuring the frequency characteristics of the cable with discrete tones; (b) measuring the cable noise and interference for every sub-channel of the whole bandwidth; (c) determining the theoretical and practical data rates for every sub-channel, based on the measured frequency characteristics and noise characteristics, as well as modem parameters selected by the user; and (d) estimating the theoretical and practical data rates for the ADSL bandwidth based on the ADSL DMT standard selected by the user.

Abstract: A method of estimating loss of a splice between first and second optical fibers spliced together by a mechanical splice or a mechanical splice-based connector having a portion through which light leaked from the splice can emerge comprises the steps of launching light into the first fiber, collecting light leaked from the portion using a multiplicity of ports, measuring the power level of the collected light and deriving the insertion loss from the measured power level, a predetermined power level and a collection factor C representing a relationship between measured power level, predetermined power level, power level of light in the first fiber, and power level of light coupled into the second fiber previously obtained for the same or similar connecting device. The ports may be angularly spaced about the axis of the connector or splice. The predetermined power level may be obtained by measuring light emerging from the connector but without the second fiber in position.

Abstract: A widely-tunable laser apparatus comprises a plurality of tunable lasers having different ranges that overlap to encompass a desired operating range of wavelengths (for example from 1250 nm to 1650 nm) of the widely-tunable laser apparatus. The tunable lasers are tunable synchronously and selectively with their respective outputs connected in common to sweep the output of the widely-tunable laser apparatus substantially continuously over said operating range. The tunable lasers share the same tuning means which has a plurality of independent channels, each for light from a respective one of the tunable lasers.

Abstract: Apparatus for measuring polarization mode dispersion (PMD) of a device, e.g. a waveguide, comprises a broadband light source (10,12) for passing polarized broadband light through the device (14), an interferometer (20) for dividing and recombining light that has passed through the device to form interferograms, a polarization separator (30) for receiving the light from the interferometer and separating such received light along first and second orthogonal Feb. 25, 2003 Feb.

Abstract: An instrument for measuring bidirectional optical signals propagating in an optical transmission path between elements one of which will not transmit if continuity of the transmission path is not maintained, for example a branch path between a central offices optical line terminal (OLT) and an end-user's optical network terminal (ONT), comprises first and second connector receptacles for connecting the instrument into the path, a 2×2 coupler (32) having first and second ports (28, 30) connected to the first and second connectors (22, 24), respectively, for completing the optical transmission path, a third port (36) for, outputting a portion of each optical signal received via the first port (28) and a fourth port (34) for outputting a portion of each optical signal received via the second port (30) Detectors (38, 42, 44) coupled to the third and fourth ports convert the optical signal portions into corresponding electrical signals, which are processed to provide the desired measurements.

Abstract: A measurement system for wide dynamic range operation in optical power measurement applications is disclosed. The system provides amplification over a wide dynamic range without glitch, signal loss or transient effect during range switch. This is achieved by combining first and second multi-gain photodiode amplifiers (10/1,10/2), with their input ports connected in common to receive the input signal from a photodiode, and by switching their gain settings to get continuous readings of a photodiode signal. More specifically, the gain setting of the first amplifier is changed only when the signal from the other amplifier is being output, and vice versa. Such an amplification system ensures a smooth transition within a wide dynamic range while monitoring various signal levels, e.g., monitoring processes with substantial variations of optical power from very high levels to very low levels.

Abstract: A polarization independent optical spectrum analyzer comprises a diffraction grating, input and output slit means and polarization-maintaining multi-pass optics for directing a light beam to and fro across the diffraction grating while maintaining its linear state of polarization. The optical spectrum analyzer further comprises a polarization control unit for decomposing a light beam for analysis into first and second beams having mutually orthogonal states of polarization (SOPs) and then adjusting one or both SOPs so that they are parallel to each other and to one of the dispersion plane of the diffraction grating which is rotated to select different wavelengths of the first and second light beams. The first and second light beams are passed across the diffraction grating repeatedly by multi-pass polarization-maintaining optics and are outputted and detected separately.

Abstract: A uniform optical attenuation system for high power detection comprises a detector unit (11), a plurality of diffusing elements (14, 15, 16, 17) for receiving and diffusing input light and an apertured plate (18, 19) for limiting light received by the detection unit to a portion only of the light diffused by the diffusing arrangement. The plurality of diffusing elements comprise a first diffusing element (12) for receiving and diffusing an input light beam to provide a distributed light source for a subsequent one of the diffusing elements, and final one of the diffusing elements which directs diffused light towards the detection unit. The light detected by the detection unit (11, 18, 19) is substantially independent of variations in numerical aperture of the input light. Each diffusing element may be a diffusing surface, such as a ground glass or opal diffuser, which renders the system substantially independent of variations in optical properties due to heating effects caused by the high energy light.

Abstract: A dual-channel, double-filtering, multi-pass OSA having a narrow spectral linewidth response and high ORR comprises a diffraction grating (DG), two input ports (P1?, P1?) for directing first and second input light beams (LR, LT) onto the grating; a retroreflector (RAM1) for returning the dispersed light beams to the grating for dispersion again; two intermediate output ports (P2?, P2?) for receiving the twice-dispersed light beams; two secondary input ports (P3?, P3?) coupled to the intermediate output ports by polarization-maintaining waveguides (PMF2?, PMF2?) for directing the light beams onto the grating a third time, with their SOPs having a predetermined orientation relative to the SOPs of the first and second light beams when first incident upon the grating, the retroreflector (RAM1) returning the three-times-dispersed light beams to the grating for dispersion a fourth time; and two output ports (P4?, P4?) for receiving the light beams after dispersion the fourth time.

Abstract: Apparatus for making wavelength-resolved polarimetric measurements comprises an interferometric source (10,12), for example a broadband source (10) and an optical interferometer unit (12), and a polarization generator unit (16) for generating different states of polarization of light received from the interferometric source and applying same to a device-under-test (30). A polarimeter unit (20) receives and polarimetrically-analyzes light from the device-under-test, converts the polarimetrically-analyzed light into electrical signals, and, using Fast Fourier Transform numerical analysis, computes therefrom the wavelength-resolved polarimetric measurements. Placing the optical interferometer unit (10,12) “upstream” not only of the device-under-test (30), but also of the polarisation generator unit (16), means that the latter substantially eliminates polarization dependent effects introduced by the former.

Abstract: A polarization independent optical spectrum analyzer comprises an inherently polarization sensitive angle-tuned filter element and polarization-maintaining multi-pass optics for directing a light beam to and fro through the tunable filter element while maintaining its linear state of polarization. The optical spectrum analyzer comprises such a tunable optical filter in combination with a polarization control unit for decomposing a light beam for analysis into first and second beams having mutually orthogonal states of polarization (SOPs) and then adjusting one or both SOPs so that they are parallel to each other and to one of the principal axes of the angle-tuned filter which selects different wavelengths of the first and second light beams. The first and second light beams are passed through the filter repeatedly by multi-pass polarization-maintaining optics.

Abstract: A property of a device that is dependent upon both wavelength and state of polarization is measured by; passing through the device an optical signal having its wavelength and SOP varied, the wavelength over a spectral range of the device and the SOP between four Mueller SOPs; measuring the insertion loss of the device for each of the four SOPS and at each wavelength; using the four insertion loss measurements for each of the four different states of polarization for each wavelength to compute the four elements of the first line of the Mueller matrix for each wavelength; and using the Mueller matrix elements, computing insertion loss variations for the device for a multiplicity of input states of polarization in addition to the four states of polarization for which the actual attenuation measurements were made and using the insertion loss variations to compute the polarization and wavelength dependent property.

Abstract: In a method of polarization optical time-domain reflectometer (P-OTDR) for measuring a parameter of an optical transmission path, for example an optical fiber, by transmitting pulses of light into the path and measuring the state of polarization of backscattered light against distance, statistical analysis of the degree of polarization (DOP) of the backscattered signal is used to detect sections of an optical transmission path for which mode-coupling behaviour (long h) leads to high PMD. Preferably, successive DOP measurement are taken with different state of polarization, SOP, for the P-OTDR light source.

Abstract: An optical spectrum analyzer comprises a diffraction grating (DG), a polarization decomposing unit (PDM) for decomposing the input light beam into first and second light beams having mutually-perpendicular linear states of polarization, and two output ports (FP2/1, FP2/2) each for receiving from the grating, substantially exclusively, a respective one of the polarized light beams (LT, LR) after diffraction by the diffraction grating (DG).

Abstract: A photodetector device comprising a photosensitive detector (12; 96) and one or more interfaces (20′, 20″, 28, 98) between dissimilar media is configured so that a light beam (LB) for detection will pass through the interface(s) along a beam axis that is not normal to the interface(s). The deviation (&thgr;) from the normal will be such that polarization dependent transmission introduced at the interface(s) will compensate for inherent polarization dependency of the detector (12; 96). The deviation may be achieved by inclining the interface(s) relative to a predetermined direction along which the light beam will be incident. Where the photosensitive detector is in a housing (84) with a window (20) through which the light beam enters the housing, the housing can be tilted. In such as case, there are three interfaces, one (28; 98) at the surface of the detector (12; 96), and one at each surface (20′, 20″) of the window (20).

Abstract: An adapter for receiving optical fiber connectors, of the type having an inner part suitable for receiving an internal connector carrying an internal optical fiber having an end ferrule, and having an outer part suitable for receiving an external connector having an external optical fiber with an end ferrule, is made suitable for use with different external connectors by having the outer part readily separable from the inner part. The parts have complementary engaging surfaces and cooperating engaging means which lock the two parts together upon relative rotation of one of the parts relative to the other through about a right angle. The usual alignment sleeve is carried by the outer part, and when this part has been separated from the inner part the internal fiber ferrule is accessible for cleaning.

Abstract: In order to avoid errors inherent in the measurement of electrical phase differences or pulse arrival time in relative group delay measurements, different optical signals have their intensity modulated at a common high frequency and different permutations are selected. The amplitudes of corresponding electrical signals are detected and phase differences are computed on the basis of trigonometrical relationships. Because the modulation frequency is known, time differences can be deduced. Apparatus for measuring the phase differences conveniently comprises a slotted wheel (26) which passes selected ones or both of the optical signals. One of the optical signals may be split to produce a third signal with a predetermined phase shift, e.g. about 90 degrees at the modulation frequency and the amplitudes of some possible permutations of the three optical signals used to compute the phase difference. The measurements may be used to compute chromatic dispersion, polarization mode dispersion, elongation, and so on.

Abstract: An optical spectrum analyzer comprises a diffraction grating (DG), a polarization decomposing unit (PDM) for decomposing the input light beam into first and second light beams having mutually-perpendicular linear states of polarization, and two output ports (FP2/1, FP2/2) each for receiving from the grating, substantially exclusively, a respective one of the polarized light beams (LT, LR) after diffraction by the diffraction grating (DG).

Abstract: Apparatus for measuring very low levels of polarization mode dispersion of optical devices, that is inexpensive, robust and portable, comprises a broadband source and a polarizer for directing substantially completely polarized broadband light into the device under test with the polarization in a plane substantially perpendicular to the propagation direction of the light. Light leaving the device is analyzed spectrally to produce a spectrum of intensity in dependence upon wavelength or frequency of such light for each of at least two mutually orthogonal polarization axes in a plane perpendicular to the propagation axis of the light leaving the device. The spectra are used to compute Stokes parameters s1, s2 and |s3| for each of a plurality of wavelengths within the bandwidth of the broadband light.