Abstract: A receiver for an OTDM/PDM pulse train (10) in which the pulses (12) have alternating polarizations (P1, P2) has a polarization insensitive optical switch (16; 161, 162, 163, 164) for isolating optical pulses (10?) within the pulse train (10), and a polarization selective element (17) for separating from the isolated pulses (10?) at least one component that has a single polarization. This allows to considerable relax the constraints posed on the switch since components in the isolated pulses that result from interchannel interference can, at least to a large extent, be eliminated by the subsequent polarization selective element (17).

Abstract: An optical receiving apparatus sets, efficiently and optimally, a delay interferometer and a variable wavelength dispersion compensator in the apparatus.

Abstract: In an over-sampled maximum-likelihood sequence estimation (MLSE) receiver system, the optimal sample spacing is determined for a variety of conditions. In an illustrative implementation, the system includes an optical filter for tightly filtering an incoming optical data signal with an on-off-keying (OOK) non-return-to-zero (NRZ) format, followed by an optical-to-electrical converter, an electrical filter, a sampler, and a MLSE receiver. The sampler samples the filtered electrical data signal twice each bit period with unequal sample spacings. For wide optical filtering bandwidths, the optimal sample spacing occurs at less than 50% of a bit period. For narrow bandwidths, the optimal sample instances occur closer to the maximum eye opening.

Abstract: Methods and systems for PMD compensation in an optical communication system are implemented by transmitting multiple optical signals through a common optical conduit to an optical compensator that adjustably rotates the polarization states of the multiple optical signals and transmits the rotated optical signals to an optical receiver. The receiver, upon sensing an excessive error condition, commands the optical compensator to change the polarization state of rotation, which changes the PMD profile of the received optical signals.

Abstract: A system and method for superheterodyne detection in accordance with the invention. The system comprises a first conversion unit for performing a first heterodyne operation on an optical input signal to generate an electrical IF signal. A second conversion unit is electrically or optically coupled to the first conversion unit. The second conversion unit performs a second heterodyne operation to generate an electrical output signal suitable for signal processing.

Abstract: An optical phase locked loop (1), comprising an optical phase detector (2) receiving as inputs an optical signal to be locked (symbol I) and a locked optical signal (symbol II) and providing as its output an electrical error signal (VPD) indicating the phase shift existing between the optical signal to be locked (symbol I) and the locked optical signal (symbol II); an electrical loop filter (3) receiving the electrical error signal (VPD) and outputting a filtered electrical error signal (VPDF), and an optical voltage controlled oscillator (4) receiving as an input the filtered electrical error signal (VPDF) and outputting the locked optical signal (symbol II).

Abstract: A method of, and an apparatus for, biasing photodiodes used in optical receivers or optical transceivers includes an electronic circuit for generating a bias voltage for the photodiode and a processor having a pulse-width modulation unit that produces pulse-width modulated pulses at a predetermined number of duty cycle values M. The processor is coupled to the electronic circuit such that a magnitude of the bias voltage is dependent upon a duty cycle value of at least one of the pulse-width modulated pulses. Advantageously, the processor includes a program for modulating the pulse-width modulated pulses generated by the processor between two different duty cycle values such that a number of obtainable bias voltages is greater than M. The result is a significant reduction in the number of necessary components.

Abstract: A system and method for detecting digital symbols carried in a received optical signal. The system comprises a functional element operative to receive a stream of samples of an electrical signal derived from the received optical signal and to evaluate a non-linear function of each received sample, thereby to produce a stream of processed samples. The system also comprises a detector operative to render decisions about individual symbols present in the received optical signal on the basis of the stream of processed samples. In an embodiment, the non-linear function computes substantially the square root of each received sample.

Abstract: An optical receiver having compensation for signal level transients. The receiver includes an OE converter, a transient threshold compensator, a threshold combiner and a CDR system. The OE converter receives the incoming optical signal and provides a modulated electrical signal having a high speed response for tracking the modulation on the optical signal and an averaged electrical signal having a moderate speed response for tracking changes in the average level of the optical signal. The transient threshold compensator processes the averaged electrical signal for providing a transient feedforward adjustment. The threshold combiner combines the transient feedforward adjustment with a lower speed BER feedback threshold adjustment for providing a decision threshold signal. The CDR system uses the decision threshold signal for recovering a clock, providing the BER feedback threshold adjustment, and estimating the data carried by the modulation.

Abstract: An optical receiver circuit is constructed to be immune to interference from external interference signals. The optical receiver circuit includes a differential amplifier having an optical reception device connected to one input of the differential amplifier. The optical receiver circuit also includes an electrical element for simulating the electrical behavior of the reception device in the illumination-free state. The electrical element is connected to the other input of the differential amplifier.

Abstract: In an adaptive optics module, wavefront sensing and data detection are implemented in a single device. For example, an optical-to-electrical converter converts a data-encoded optical beam to an intermediate electrical signal, which contains both the data encoded in the beam and also wavefront information about the beam. The data and wavefront information are later separated, for example by frequency filtering.

Abstract: An optical coherent receiver in one embodiment has a heterodyne configuration, and in another embodiment has a homodyne configuration, in each configuration employs multiple feedback signaling and analog/digital processing to optimize response to a modulated optical input signal, the provision of both individual RF I and RF Q channel outputs.

Abstract: A transmission apparatus that receives an optical signal by selecting any one of a plurality of provided optical signal transmission paths through protection control is configured to include a plurality of optical signal outputting sections that output the optical signals transmitted through said optical signal transmission paths respectively as optical signals having wavelengths that are different from each other, a wavelength selective optical switch capable of selectively outputting light of a wavelength corresponding to any one of the optical signals coming from the optical signal outputting sections on the basis of the frequency of a controlling frequency signal, and an optical switch controlling section that supplies said controlling frequency signal to the wavelength selective optical switch so as to output the optical signal coming from the optical signal transmission path side that is selected by said protection control among the optical signals coming from the optical signal outputting sections.

Abstract: An optical receiver is disclosed comprising an erbium-doped fiber amplifier (EDFA) that is coupled to a photodiode and transimpedance amplifier without filtering output light signal in the EDFA. Optionally, a clock/data regenerator can be coupled to the electrical output of the transimpedance amplifier for compensating for noise distortion and timing jitter for affecting the control loop feeding back for adjusting the electrical current into a pump laser of an optical pre-amplifier. Furthermore, the optical receiver of the present invention can also be implemented in a transponder.

Abstract: The invention discloses a digital adjusting method for an optical receiver module, and the method implements real-time monitor of parameters, on-line adjustment and non-linear compensation. The digital optical receiver module includes elements as follow: a voltage output circuit of optical power detection 24, a DC/DC voltage boost circuit 22 and a bias voltage adjusting unit which is consisted of an optical-electronic conversion circuit 21; a digital adjusting unit 25, an analog-digital converter (A/D converter) 26 and a memory 27. The digital adjusting unit 25 makes on-line adjustment and implements temperature compensation and dark current compensation of the optical receiver module. The A/D converter 26 monitors the optical power, the working temperature and the bias voltage of the optical detector in real time. The memory 27 stores parameters of the optical receiver module for comparison with a detected optical power and measured temperature etc. and for on-line interrogation.

Abstract: The inventors propose herein a switch fabric architecture that allows broadcasting and fast channel access in the ns-range. In various embodiments of the present invention, 10 Gb/s receiver modules are based on a novel heterodyne receiver and detection technique, which is tolerant to moderate wavelength drifts of a local oscillator. A gain clipped electrical amplifier is used in the novel receiver as a rectifier for bandpass signal recovery.

Abstract: An optical receiver includes a light-receiving part and a control part. The light-receiving part includes an APD, a PIN-PD, and a branching optical device. First signal light is incident on the light-receiving part and is divided into second signal light and third signal light which are incident on the APD and the PIN-PD, respectively, by the branching optical device. Due to this structure, the third signal light is incident on the PIN-PD without the quantity thereof being varied depending on the polarization state of the first signal light. The control part generates a supply voltage at which a desired avalanche multiplication factor is obtained in the APD on the basis of the output current from the PIN-PD. According to the above-described structure, the avalanche multiplication factor of the APD is accurately controlled on the basis of the output current of the PIN-PD.

Abstract: The discrimination phase margin monitor circuit (10) of the present invention comprises a first discrimination circuit (11 and 12) discriminating an input data signal using a clock signal extracted from the input data signal, a second discrimination circuit (13 and 14) discriminating the input data signal using a clock signal with a frequency different from that of the clock and an operation circuit (15 and 16) calculating the exclusive OR of the output signal of the first discrimination circuit and that of the second discrimination circuit and obtaining a phase margin monitor output signal by averaging the exclusive ORs.

Abstract: There is provided an optical receiver in which a time gate can be realized with a simple, low-cost configuration, and which has few aspects that require adjustment. The optical receiver of the present invention comprises a decoding circuit in which optical signals which are spread over time in accordance with a coding pattern are inputted and decoded in accordance with a decoding pattern; and a time gate circuit for generating a time gate signal that represents the interval of time in which a significant optical pulse is present in the decoded optical signal, and controlling the passage of the decoded optical signal.

Abstract: An optical dispersion monitoring apparatus and an optical dispersion monitoring method are capable of monitoring dispersion accurately with a simple construction in an optical transmission system using the same. To this end, the optical dispersion monitoring apparatus includes a light receiving section converting an input optical signal into an electrical signal, a signal transition position detecting section detecting the voltage level of a waveform of the output signal from the light receiving section, at a crossing point of a rising edge and a falling edge, and a cumulative dispersion information extracting section comparing the voltage level at the crossing point with a reference signal to extracts cumulative dispersion information.

Abstract: The invention relates to an optical receiving station, an optical communication system, and a dispersion controlling method for precisely controlling chromatic dispersion in an optical transmission line or chromatic dispersion in an optical transmission line that varies with time. An optical receiving station is provided with a dispersion compensating section for receiving, via an optical transmission line, an optical signal modulated according to an optical duo-binary modulation method and for changing a dispersion value to be used for compensating for chromatic dispersion in an optical transmission line, an intensity detecting section for detecting the intensity of a specific frequency component of the optical signal output from the dispersion compensating section, and a controlling section for adjusting the dispersion value of the dispersion compensating section so that the output of the intensity detecting section has a predetermined extreme value.

Abstract: A method is provided for an equalization strategy for compensating channel distortions in a dual-polarization optical transport system wherein the received signal includes a complex signal of a first transmitted polarization component and a complex signal of a second transmitted polarization component. In a first step, a blind self recovery mode used a blind adaptation algorithm in calculating and modifying multiple complex equalizer transfer function coefficients to enables recovery of only the complex signal of the first transmitted polarization component. In a second step, equalization is performed in a training mode for recovery of the complex signals of the first and second transmitted polarization components. In a third step, equalization is performed in a data directed mode. The method is suited for a digital signal processing implementation in a coherent receiver.

Abstract: The present invention provides an optical receiver that uses an avalanche photodiode (APD) whose multiplication factor m is controlled to compensate the temperature dependence thereof. An optical module of the present invention includes a light-receiving device in addition to the APD. The light-receiving device may be a semiconductor thin film or a PIN photodiode, and is disposed in front of the APD. Accordingly, the light-receiving device receives a portion of signal light, and transmits a rest portion thereof. The APD receives the rest portion of the signal light. The bias voltage applied to the APD is so controlled that a first photocurrent generated in the light-receiving device and a second photocurrent generated in the APD maintain a constant ratio.

Abstract: A transimpedance amplifier having adjustment for optical distortion in an optical communication link. The transimpedance amplifier comprises a transimpedance stage and a post amplifier stage, which has a feedback path including an optical distortion adjustment circuit.

Abstract: In an optical transmitting or receiving apparatus, a plurality of replaceable electro-optic converters or a plurality of replaceable opto-electric converters are provided in correspondence with optical transmission lines.

Abstract: An optical receiver includes a PIN photodiode (PIN-PD) having an incident surface for receiving signal light, the PIN-PD transmitting a part of the signal light to the surface opposite to the incident surface, and an avalanche photodiode (APD) having an incident surface for receiving light transmitted through the PIN-PD. In the optical receiver, the ratio of the quantity of signal light detected by the PIN-PD and the ratio of the quantity of signal light detected by the APD are not affected by the polarization state of the signal light incident on the optical receiver, and accordingly the avalanche multiplication factor of the APD is suitably controlled on the basis of the signal light detected by the PIN-PD.

Abstract: The present invention specially deals with a transmission system in which at least two coded optical signals are multiplexed and transmitted between an emitter and a receiver, a first of said coded signals providing a first spectrum having a first carrier wavelength and two side-bands, a second of said coded signals providing a second spectrum having a second carrier wavelength higher than said first carrier wavelength and two side-bands. This transmission system comprises at least one optical filter with to combined filters for the carrier wavelength and the respective sideband.

Abstract: Methods and systems for PMD compensation in an optical communication system are implemented by transmitting multiple optical signals through a common optical conduit to an optical compensator that adjustably rotates the polarization states of the multiple optical signals and transmits the rotated optical signals to an optical receiver. The receiver, upon sensing an excessive error condition, commands the optical compensator to change the polarization state of rotation, which changes the PMD profile of the received optical signals.

Abstract: A method and system for multi-level power management in an optical network is provided. They include three levels of power equalization. The first level of power management equalizes the powers of channels in a band of channels. The second level of power management equalizes the average powers of bands of channels on a fiber. The third level of control equalizes the powers of bands of channels on working and protection fibers for a path in the optical network. As a result, this multi-level power management in the network provides a dynamic, automatic method for the network to adjust to changing operating conditions and configurations and to maintain relatively stable network powers. Each level of power management may be implemented jointly or independently, and operates autonomously so that, for example, one modification comprises continuous first level control and only periodic second level control.

Abstract: A wavelength beam splitter (WBS) is combined with a conventional tunable filter to sequentially select different channels in a multi-channel communication system. The WBS is characterized by a periodic spectral response with period equal to the channel spacing of the ITU grid. Preferably, the WBS consists of an optical cavity with an optical path length that produces a free-spectral range substantially equal to the channel spacing of the ITU grid. The WBS is used to separate the signal in each channel passband from the noise in the corresponding channel stopband. This provides a signal and a noise output for each channel. A tunable filter is used to scan the channels of the ITU grid sequentially and provide output signals for a single channel at a time. Therefore, a much smaller frequency bandwidth needs to be measured at a time and a less expensive detector may be used.

Abstract: In a receiver operable in response to a data signal, an eye aperture size is detected along a time axis by an eye aperture detection circuit and is controlled by a control circuit so that it becomes a maximum. The eye aperture detection circuit determines different decision time points of the same level arranged along a time axis and judges whether or not an error is caused to occur at each of the decision time points, so as to detect the eye aperture size and to produce detection results. The control circuit processes the detection results in accordance with a predetermined algorithm to successively vary the eye aperture size and to keep the data signal at an optimum amplitude.

Abstract: An optical receiver includes a first light receiving element to convert an optical signal to an electric signal and to output the electric signal from one end. A light receiving element row is connected to the other end of the first light receiving element to supply electric power to the first light receiving element. The light receiving element row includes a plurality of second light receiving elements connected in series.

Abstract: A transmitted optical signal is first subjected to polarization mode dispersion compensation by a polarization mode dispersion compensator (PMDC), and then, its chromatic dispersion is compensated by a variable chromatic dispersion compensator (VDC) after the polarization mode dispersion compensation. How much the optical transmission signal suffers from polarization mode dispersion, which is needed to perform the polarization mode dispersion is measured using a Stokes parameter that is not affected by chromatic dispersion.

Abstract: An Improved Signal Receiver Having Wide Band Amplification Capability is disclosed. Also disclosed is a receiver that is able to receive and reliably amplify infrared and/or other wireless signals having frequency bandwidths in excess of 40 MHz. The receiver of the present invention reduces the signal-to-noise ratio of the received signal to ?th of the prior systems. The preferred receiver eliminates both the shunting resistor and the feedback resistor on the input end by amplifing the signal in current form. Furthermore, the receiver includes transconductance amplification means for amplifying the current signal without the need for Cascode stages. Finally, the receiver includes staged amplification to amplify the current signal in stages prior to converting the signal into a voltage output.

Abstract: A photomultiplier module (PMT), preferably a PMT with a gallium arsenide (GaAs) photocathode, is used as a N-photon detector (N is an integer ?2). The PMT detects the N-photon absorption rate of an optical signal having a wavelength range extending from 1.0 ?m to an upper wavelength region that increases as the number of photons simultaneously absorbed by the PMT increases beyond two. The N-photon absorption rate is used by a signal compensation apparatus to reduce impairments which affect the rate, such as group velocity dispersion and/or polarization mode dispersion, in a received optical pulse communication signal. The N-photon absorption rate can also be used to determine the optical signal-to-noise ratio of a received optical pulse communication signal, and/or to synchronize a second optical pulse signal with the first optical signal.

Abstract: Systems and methods for regulating optical power are described. A system for regulating optical power includes a laser driver circuit that receives an enable/disable signal and a data modulator input. The enable/disable signal regulates asynchronous mode operation. The system also includes a laser module including a laser diode emitter and a photodiode detector. The laser module is coupled to the laser driver circuit and receives a laser bias current from the laser driver circuit. The system also includes a switch coupled to the photodiode to receive a signal from the photodiode detector. The system also includes an automatic power control (APC) feedback circuit that receives a signal from the switch and provides a laser bias current feedback signal to the laser driver circuit to compensate for power output changes in the laser diode emitter over time.

Abstract: An amplitude modulated optical communication system and method are disclosed that achieve bandwidth compression by making use of an n level amplitude modulation scheme in one or more frequency bands. A soft decision decoder is disclosed that provides at least two soft slicing levels between each signal level in the multiple level transmission scheme to define an “uncertainty” region therebetween. The soft slicing levels are used to evaluate the reliability of a given bit assignment. In addition to assigning a digital value based on the received signal level, one or more soft bits are assigned indicating a “reliability” measure of the output code.

Abstract: A tunable dispersion compensator whose passband center wavelength changes when the amount of dispersion compensation is changed is suitably adjusted. The relationship between temperature for keeping the center wavelength constant and the amount of dispersion compensation is stored in advance. After controlling the amount of dispersion compensation to achieve best or optimum transmission quality, the amount of dispersion compensation is converted into temperature in accordance with the stored relationship and, based on that, the temperature is controlled to keep the center wavelength constant.

Abstract: The output monitor/control device is provided with: Mach-Zehnder circuit 104 that receives a light beam, branches the light beam into two light beams having a phase difference of 180°, and transmits each of the light beams, exhibiting a periodic optical transmittance-optical frequency characteristic a period of a frequency interval corresponding to a predetermined free spectral range; first and second photodiodes that each receive one of two light beams supplied from the Mach-Zehnder circuit; calculation circuit 108 that calculates a predefined discrimination formula for evaluating wavelength change of the light beam based on output currents of the photodiodes; and wavelength control circuit 111 that detects change in wavelength based on the calculation results by the calculation circuit and adjusts the wavelength to a set value.

Abstract: A system that transmits amplitude modulated data in a wavelength-encoded format and then uses a wavelength-sensitive receiver to convert the received optical signal back to the original amplitude modulated data. This system enables transmission of optical signals that are less sensitive to attenuation and attenuation changes. This system is applicable to data in digital, multilevel, or analog formats.

Abstract: The present invention implements open fiber control in an optical transceiver. During normal operation, the transceiver transmits signals through a connection to an optical network. When the connection is intact, the total power transmitted can be greater than the eye-safety limit. When the connection breaks, the transceiver detects the loss of signal and disables transmissions over all channels except for one. The transceiver continues transmission on the single enabled channel at an eye-safe level. When the connection is fixed and a signal reappears, the transceiver detects the signal reappearance and re-enables all channels that had previously been disabled. By allowing the transceiver to transmit with greater optical power when the connection is intact, an increased data rate and longer transmission distances can be achieved. At the same time, safety is preserved, because transmissions drop to acceptable power levels whenever a break is detected.

Abstract: An optical receiver may include a photodetector that is configured to generate an electrical signal in response to detecting an optical signal, a first gain path coupled to receive and amplify the electrical signal generated by the photodetector, and a second gain path coupled to receive and amplify the electrical signal generated by the photodetector. The second gain path may have a higher gain than the first gain path. The two gain paths may be configured to amplify the electrical signal independently of each other. The optical receiver may also include a switch that selectably outputs an amplified electrical signal from one of the gain paths based on a magnitude of a signal corresponding to the optical signal.

Abstract: A method of splitting an optical data signal starting with an optical data signal split into n equal data sub-signals, and an optical binary auxiliary signal with the same bit rate and phase is added to each optical data sub-signal. Each nth bit of the optical binary auxiliary has a higher level, with the phases of each of the n optical auxiliary signals showing a relative displacement of one bit. As a result, n aggregate signals are generated, each of which is fed to a decision-maker. A decision-maker threshold is set above the amplitude of the data sub-signal and below the amplitude of the aggregate signal. Each decision-maker, therefore, emits an electrical data signal with 1/n times the bit rate of the data signal.

Abstract: An optical receiver photonic integrated circuit (RxPIC) system includes a monolithic semiconductor chip having an input to receive a WDM combined channel signal comprising a plurality of optical channel signals of different wavelengths. A chip-integrated decombiner is coupled to the chip input to receive the WDM combined channel signal and separate the same into a plurality of different channel signals having different wavelengths. An array of integrated photodetectors, also integrated on the chip, each receive a separated channel signal and together provide a plurality of electrical signals representative of the optical channel signals. An electronic amplifier receives and amplifies the electrical signals. An electronic dispersion equalization (EDE) circuit is coupled to receive and adjust the amplified electrical signals for timing errors due to imperfect clock recovery of said electrical signals. An clock and data recover (CDR) circuit recovers a signal clock and data signals from the electrical signals.

Abstract: A dispersion compensation controlling apparatus used in a very high-speed optical communication system adopting optical time division multiplexing system comprises a first specific frequency component detecting unit (2a) detecting a first specific frequency-component in a baseband spectrum in a transmission optical signal inputted to a receiving side over a transmission fiber as a transmission line (6a), a first intensity detecting unit (3a) detecting information on an intensity of the first specific frequency component detected by the first specific frequency component detecting unit (2a and a polarization-mode dispersion controlling unit (220a) con trolling a polarization-mode dispersion quantity of the transmission line (6a) such that the intensity of the first specific frequency component detected by the first intensity detecting unit. (3a) becomes the maximum, thereby easily detecting and compensating polarization-mode dispersion generated in a high-speed optical signal.

Abstract: A method of decoding a signal in an optical fiber. In one embodiment the method includes receiving the optical signal, wherein the optical signal is a pulse amplitude modulated signal. Converting the optical signal to an electrical signal. Comparing the electrical signal with a plurality of levels. Producing comparison output signals based on the comparison of the electrical signal with the plurality of levels. Processing the comparison output signals on a clock to produce processed output signals and latching the processed output signals on a clock signal to generate the plurality of serial, digital data streams.

Abstract: An optical transmission system including at least one optical transmitter configured to transmit at least one signal wavelength and a tuning wavelength, an optical receiver including an optical filter having a filter bandwidth including the at least one signal wavelength and a percentage of the tuning wavelength and an optical to electrical signal converter configured to receive the at least one signal wavelength from said filter, a first tuning optical to electrical converter configured to receive a first portion of the tuning wavelength stopped by said filter, a second tuning optical to electrical converter configured to receive a second portion of the tuning wavelength passed by said filter, and a filter controller configured to tune the filter bandwidth based on the relative proportion of first and second portions of the tuning wavelength provided to the first and second tuning optical to electrical converters.

Abstract: An Improved Signal Receiver Having Wide Band Amplification Capability is disclosed. Also disclosed is a receiver that is able to receive and reliably amplify infrared and/or other wireless signals having frequency bandwidths in excess of 40 MHz. The receiver of the present invention reduces the signal-to-noise ratio of the received signal to ?th of the prior systems. The preferred receiver eliminates both the shunting resistor and the feedback resistor on the input end by amplifying the signal in current form. Furthermore, the receiver includes transconductance amplification means for amplifying the current signal without the need for Cascode stages. Finally, the receiver includes staged amplification to amplify the current signal in stages prior to converting the signal into a voltage output.

Abstract: An optical receiver circuit including a plurality of PIN diodes, each associated with a dedicated element transimpedance amplifier, the outputs of the element transimpedance amplifiers being connected to a summing amplifier which sums the voltages output from the element transimpedance amplifiers. The optical receiver circuit provides the same output voltage value as a single large PIN diode having an active area comparable to the sum of the active areas of the smaller PIN diodes, and thus has the same high sensitivity as the single large PIN diode but a much wider bandwidth.

Abstract: An increase in the code error rate in a received signal is detected in a light receiver. An loss of signal detector circuit for a light receiver 20 comprises a main discrimination circuit 8a for comparing the intensity of an electric signal obtained by converting a light signal by photoelectric conversion using a photoelectric conversion element 1 with a predetermined discrimination threshold; a reference discrimination circuit (8b, 8c, . . . ) for comparing the intensity of the electric signal with regard to a reference threshold which differs from the discrimination threshold; and an operation circuit 9 for detecting loss of the signal based on the results of the comparisons performed by the main discrimination circuit 8a and the reference discrimination circuit (8b, 8c, . . . ).