Abstract: An apparatus and method for filtering an optical input is disclosed. In an illustrative embodiment, an optical input is split into polarization components along separate paths. The polarization components are then fed into a first electro-optic device that includes a set of electrodes across which a voltage is applied to adjust a wavelength transmission characteristic of the device. A section of the first device positioned between the electrodes preferably has a birefringence that is adjusted depending on the voltage applied across the electrodes. The adjusted components of the optical input are thereafter combined to produce an optical output. Accordingly, the optical input can be attenuated based on the voltage applied to electrodes of the first electro-optic device.

Abstract: A method and apparatus accommodate for wavelength drift of optical sources. The apparatus includes an optical source having a wavelength control port and an output port, providing one or more optical signals that drift at a first rate. A signal generator within the apparatus provides a control signal to the wavelength control port of the optical source that causes wavelength variations to be superimposed on the wavelength drift of the one or more optical signals. The superimposed wavelength variations occur at a second rate that exceeds the first rate. The apparatus also includes a receiver coupled to the output port of the optical source, that receives the one or more optical signals having the superimposed wavelength variations. The receiver accommodates for the wavelength variations caused by the control signal, enabling a received wavelength to be distinguished from the drifting wavelength of the optical source. A corresponding method accommodates for wavelength drift of optical sources.

Abstract: An optical transmitter for digitally modulated RF sub-carriers comprising an electroabsorption modulator integrated distributed feedback laser for transmitting the digitally modulated RF sub-carriers in the sub-octave frequency range. The negative bias is optimized for maximum carrier signal to noise ratio (CNR) which occurs when the composite triple beat (CTB) non-linear distortion is minimum that allows higher modulation index without causing any in-band distortion. A higher CNR increases the receiver sensitivity and thus more users can be served from a single transmitter or the transmitting signal can be modulated with higher order of digital modulation.

Abstract: An optical data transmission system using a Mach Zehnder (M-Z) modulator asymmetrically modulates the optical carrier to generate control information to optimize the transmitted optical signal.

Abstract: One or more single mode waveguide devices are fiber coupled such that signals to an optical element affect the coupling of the waveguide device to one or more modulators and to an optical fiber. The optical element or additional optical elements are controlled to adjust the coupling of the waveguide device to a modulator and to an optical fiber.

Abstract: A disc serving as a spatial radiation modulator has dispersed radiation filters thereon. Each filter has a transmittance or reflectance modulation function of the form sin2(m&thgr;+p&pgr;/4), where m is a positive integer and p has one of the four values 0, 1, 2, 3. A radiation beam including selected wavelength components is diffracted into an elongated image dispersed according to wavelength. Different wavelength components are focused onto different filters on the modulator and are encoded by correspond filters. Since the modulation functions of the filters are orthogonal to one another, it is possible to extract the amplitude of each wavelength component after it has been encoded or modulated by corresponding filter from the total detected signal during one measurement.

Abstract: An optical transmitter for digitally modulated RF sub-carriers comprising an electroabsorption modulator integrated distributed feedback laser for transmitting the digitally modulated RF sub-carriers in the sub-octave frequency range. The negative bias is optimized for maximum carrier signal to noise ratio (CNR) which occurs when the composite triple beat (CTB) non-linear distortion is minimum that allows higher modulation index without causing any in-band distortion. A higher CNR increases the receiver sensitivity and thus more users can be served from a single transmitter or the transmitting signal can be modulated with higher order of digital modulation.

Abstract: An apparatus for stabilized photonic transmission is described. A light source of limited coherence length is wavelength shifted, stabilized, and data encoded to provide a stabilized photonic signal. A modulation synthesizer provides a modulation waveform embedded with the shifting, stabilization and data encoding mechanisms. A variety of modulation devices are supported. The modulation waveform is optimized for the particular modulation device. A wavelength error detector provides feedback to the modulation synthesizer. The error signal is used to stabilize the photonic signal and correct channel wavelength errors. Fixed wavelength channels and spread spectrum channels are supported.

Abstract: The optical assembly for modulating input light and providing modulated light at an output thereof includes a first arrangement, which includes a layer of a superconductive material having at least a part of the input light incident thereon as incident light. The superconductive material is switchable between a first state, in which the superconductive material exhibits a first refractive index, and a second state, in which the superconductive material exhibits a second refractive index. The first arrangement is configured to direct to the output as the modulated light a first fraction of the incident light, when the superconductive material is in the first state, and a second fraction of the incident light, when the superconductive material is in the second state, such that the modulated light exhibits a given value of extinction ratio, which is defined as a ratio of the first fraction of the incident light to the second fraction of the incident light at the output.

Abstract: An antiglare optical device for selectively attenuating bright light sources in an object field. This device comprises at the object end, the optics necessary for focusing incoming rays onto a focal plane, at the eyepiece end, the optics necessary to view the image of the object field, and at the focal plane, a photochromic layer for moderating rays originating from intense objects in the object field. A sharp focusing of the object field on the photochromic layer is achieved by means of asymmetrical optical components such as polarizing beam splitter and half lenses.

Abstract: A method for photonic wavelength shifting and stabilization is described. The method receives a photonic signal, comprised of one or more channels, and provides a wavelength shifted, stabilized, and channelized photonic signal. Data encoding is also supported. A modulation synthesizer provides a modulation waveform embedded with the shifting, stabilization and data encoding mechanisms. A variety of modulation techniques are supported. The modulation waveform is optimized for the particular modulation technique. A wavelength error detector provides feedback to the modulation synthesizer. The error signal is used to stabilize the photonic signal and correct channel wavelength errors. Fixed wavelength channels and spread spectrum channels are supported.

Abstract: The present invention aims at providing an optical transmitter utilizing a Mach-Zehnder type optical modulator, in which the optical modulator is capable of readily optimizing an optical wavelength chirp to be added to a transmission optical signal. To this end, the optical transmitter of the present invention includes a Mach-Zehnder type optical modulator having two arms driven by two drive signals, respectively, and is constituted to comprise: amplitude adjusting parts for adjusting the amplitudes of the drive signals, respectively; phase adjusting parts for adjusting the phases of the drive signals, respectively; an amplitude controlling part for feedback controlling the amplitude adjusting parts so that an amplitude ratio between the drive signals becomes a value corresponding to an optimum optical wavelength chirp amount; and a phase controlling part for feedback controlling the phase adjusting parts so that the phases of the drive signals are brought into an antiphase relation.

Abstract: An optical modulator is provided to control the intensity of a transmitted or reflected light. In a transmission mode, a separator splits arbitrarily polarized light into two polarization rays and one is made to travel a separate path from the other. A recombiner causes the two rays to recombine at an output unless an electro-optic phase retarder changes the polarization of the two rays, in which case, both of them miss the output by an amount which is a function of the voltage on the retarder. A normally-off version with low polarization mode dispersion is obtained by changing the orientation of the recombiner. A normally-on version with low polarization mode dispersion is obtained with a passive polarization direction rotator. Similar results can be obtained in a reflection mode where the input and output are on the same side of the modulator. Versions using a GRIN lens are particularly suited to modulation of light out of and back into fiber-optic cables.

Abstract: A bias control system is adapted to reduce composite second order distortions in an electro-optic modulator. The bias control system includes a pilot tone generator that generates a first pilot tone at a first frequency and a second pilot tone at a second frequency. The first and second tones are swept in frequency over a predetermined frequency range with a predetermined sweep rate, to spectrally spread third order distortion products over a larger frequency band, allowing the amplitude of the pilot tones to be increased, thereby increasing the gain of the bias control circuit.

Abstract: An adaptive pulse compressor is described, especially for use with ultrashort pulses, wherein the input pulses are modified in an iterative fashion, according to a feedback signal obtained from measurement of the output pulses. The feedback signal is programmed, by means of a programmable spatial light modulator, to allow for independent control of the individual spectral components of the incoming pulses, such that almost arbitrary phase functions can be realized to accomplish efficient compression. One of the main disadvantages associated with prior art pulse compressors, namely, the need for characterization of the uncompressed pulses, is thus eliminated. The compressor is thus capable of handling completely uncharacterized or partially characterized input pulses, or pulses from time varying sources.

Abstract: An optical modulation apparatus is provided with an optical modulator 2, a variable gain amplifying circuit 1 connected to this optical modulator 2, a detecting circuit 3 and a gain control circuit 4. A modulated input signal is entered into the variable gain amplifying circuit 1, and the variable gain amplifying circuit 1 amplifies this signal and delivers it to the optical modulator 2 as modulation drive signal. The detecting circuit 3 controls the variable gain amplifying circuit 1 on the basis of the detected signal. This optical modulation apparatus manifests steady optical modulation characteristics even if the amplitude of modulated input signals or ambient temperature varies.

Abstract: An optical output control circuit enables intensity of light signal to be kept constant, and enables various kind of additive function to be provided. A quantity of light detection circuit part includes a light-electricity conversion circuit, an A-D conversion circuit for digital converting level of analog voltage outputted from the light electricity conversion circuit. An attenuator control circuit gives attenuation quantity in accordance with value of digital control signal from CPU to an variable light attenuator. A correlation table sets relationship between digital value inputted from the quantity of light detection circuit part and an output light signal intensity (output level), and relationship between value of digital control signal and attenuation quantity of variable light attenuator beforehand.

Abstract: An optical modulator is provided to control the intensity of a transmitted or reflected light. In a transmission mode, a separator splits arbitrarily polarized light into two polarization rays and one is made to travel a separate path from the other. A recombiner causes the two rays to recombine at an output unless an electro-optic phase retarder changes the polarization of the two rays, in which case, both of them miss the output by an amount which is a function of the voltage on the retarder. A normally-off version with low polarization mode dispersion is obtained by changing the orientation of the recombiner. A normally-on version with low polarization mode dispersion is obtained with a passive polarization direction rotator. Similar results can be obtained in a reflection mode where the input and output are on the same side of the modulator. Versions using a GRIN lens are particularly suited to modulation of light out of and back into fiber-optic cables.

Abstract: An adaptive pulse compressor is described, especially for use with ultrashort pulses, wherein the input pulses are modified in an iterative fashion, according to a feedback signal obtained from measurement of the output pulses. The feedback signal is programmed, by means of a programmable spatial light modulator, to allow for independent control of the individual spectral components of the incoming pulses, such that almost arbitrary phase functions can be realized to accomplish efficient compression. One of the main disadvantages associated with prior art pulse compressors, namely, the need for characterization of the uncompressed pulses, is thus eliminated. The compressor is thus capable of handling completely uncharacterized or from time varying sources.

Abstract: An external analog modulation system is described, comprising an electro-optical modulator with a double output and a driving circuit.

Abstract: A bias voltage stabilizing method for an electricity-optical modulator based on an off-level sampling is disclosed.

Abstract: An optical modulator includes a magnetic ferrite single crystal, an optical source, a photoreceptor system and an analyzer. The magnetic ferrite single crystal has a transducer mounted thereon and arranged to receive a microwave. The microwave is modulated by a signal having a frequency lower than the microwave. The optical source is arranged such that light emitted from the optical source is introduced to the magnetic garnet single crystal and modulated by the microwave applied to the transducer. The photoreceptor system is arranged so as to receive the modulated light that is emitted from the magnetic ferrite single crystal. The analyzer is provided between the magnetic ferrite single crystal and the photoreceptor system, and the analyzer is arranged such that a rotation angle of the analyzer about an optical axis thereof is shifted by an angle in the range of about 40 degrees to about 50 degrees from an extinct position at which an amount of direct-current light transmitted through analyzer is minimized.

Abstract: A MEMS-based optical switch having improved characteristics and methods for manufacturing the same are provided. In accordance with one embodiment, an optical switch includes a single comb drive actuator having a deflecting beam structure and a mirror coupled to the actuator. The mirror is capable of being moved between an extended position interposed between waveguide channels and a retracted position apart from the waveguide channels. The actuator applies a force capable of deflecting the beam structure and moving the mirror to one of the extended positions or the retracted position and the beam structure returns the mirror to the other of the extended position or the retracted position in the absence of the application of force.

Abstract: An apparatus includes a driving voltage generator that generates a pulse driving voltage having a rising edge and a decaying edge. An optical modulator produces a first pulse at the rising edge of the pulse driving voltage and a second pulse at the decaying edge of the pulse driving voltage. Therefore, the first and second pulses are both produced from a single driving pulse voltage. The frequency of the first pulse is determined by the slope of the rising edge of the pulse driving voltage and the frequency of the second pulse is determined by the slope of the decaying edge of the pulse driving voltage. The first and second pulses are transmitted through a transmission line. A detection device detects the first and second pulses after being transmitted through the transmission line.

Abstract: A high contrast organic electroluminescent device has a transparent substrate, a transparent electrode layer, a metallic electrode layer, and an organic interlayer disposed between and in close contact with the electrode layers, the organic interlayer being comprised of an organic luminescent layer, an optional hole transport layer and an optional electron transport layer, wherein a dark conductive metal alloy layer is inserted between the organic interlayer and the metallic electrode layer.

Abstract: Disclosed is an optical writing device having an array of a plurality of writing light shutter elements for an image forming purpose. The optical writing device also has at least one first monitoring light shutter element and at least one second monitoring light shutter element. During recording period, the first second monitoring light shutter element is frequently driven, and the second monitoring light shutter element is infrequently driven. After the recording period, the first and second monitoring light shutter elements are driven with varying voltage, and light amounts from the first and second monitoring light shutter elements are sensed by a sensor. Based on the output from the sensor, optimal drive voltage for the next recording period is set.

Abstract: An optical transmitter has a Mach-Zehnder modulator, and a feedback loop that controls the bias voltage supplied to the modulator according to the insertion loss of the modulator, to compensate for DC drift in the attenuation characteristic of the modulator. The feedback loop can be switched to provide negative feedback on either the positive or negative slope of the attenuation characteristic. Switching the slope switches the polarity of the chirp generated by the modulator, enabling positive or negative chirp to be selected, whichever produces the least dispersion penalty. Since switching the slope also switches the polarity of the modulated signal, an optical transmission system using this optical transmitter has an additional circuit that selectively inverts the transmitted or received signal.

Abstract: The invention features methods and systems for processing signals by optically manipulating polaritons. In one embodiment, the signal processing method includes: converting a plurality of spatially separated input signals into polaritons that propagate in a signal processing material; optically manipulating the polaritons at a plurality of spatially separated locations in the signal processing material; and converting the manipulated polaritons into at least one output signal. In another embodiment, the signal processing method includes: converting an input signals into a polariton that propagates in a non-conductive signal processing material; optically manipulating the polariton as it propagates in the signal processing material; and converting the manipulated polaritons into an electrical output signal at an interface between the signal processing material and a conductive path in another material.

Abstract: The object of the present invention is realizing an optical fiber amplifier wherein the number of channels actually inputted to the optical fiber amplifier is read by the optical fiber amplifier itself, thereby both allowing, through self-control, optimum operating conditions in accordance with the number of transmission channels and the maintenance of transmission performance.

Abstract: A light transmitting apparatus is constructed such that a transmission data signal Sa is transmitted to a light emitting element 21 via a driving circuit 23 and a modulation circuit 24 of a control circuit 22. Signal light outputted therefrom is transmitted to a light transmission path via an optical fiber 27, an optical circulator 25 and an optical fiber 25. Stokes light (backward scattered light) produced due to stimulated Brollouin scattering is inputted to a photo detector 26 via a port P3 from a port P2 of the optical circulator 25. A detection signal Sb assuming a level corresponding to this light intensity is outputted to a modulation circuit 24 of the control circuit 22. When the level of the detection signal from the photo detector 26 increases, modulation on a driving current to the light emitting element 21 is effected so that an emission spectral width of the light emitting element 21 is enlarged, thus enlarging the emission spectral width of the light emitting element 21.

Abstract: A laser beam intensity modulation system has a variable delay circuit that is used to control the relative delay in the delivery of a video signal between two cascaded AOM's. This delay circuit is controlled by a delay controller that monitors the intensity modulation of the laser beam to provide feedback control. This system maximizes the rise and fall times of the beam to generate precise beam modulation required for high speed image setters.

Abstract: The bias point of an electro-optic modulator, such as an Mach-Zender modulator, is automatically controlled. A pilot signal, preferably two pilot tones at different frequencies, is applied to the modulator. The output of the modulator then contains various components resulting from the pilot signal. An error signal is generated, preferably coherently, based on one of these components, termed the pilot component, which preferably is located at the difference frequency of the two pilot tones. A bias signal which controls the bias point of the modulator is adjusted based on the error signal. The coherent generation of the error signal facilitates feedback loops based on phase and/or amplitude control and also supports locking the electro-optic modulator to a bias point with a preselected slope (either negatively or positively sloping).

Abstract: A deadband error reducer for a fiber optic gyroscope, which is an offset modulation signal that averages the deadband producing error over the range of the feedback modulation signal provided to the phase modulator. Although the offset modulation signal may be introduced at any of several points in the feedback loop, the offset modulation must approximate a critical amplitude at the phase modulator. If the offset modulation is a signal having segments that are linear with time, the critical amplitude of this signal will be 2.pi..

Abstract: An optical modulator for modulating light emitted from a signal light source has a substrate having electrooptic effect, an optical waveguide of a Mach-Zehnder type formed on the substrate, an electrode formed on the substrate to control light propagated in the optical waveguide, an interference light generating means for making radiation light radiated from the optical waveguide and signal light leaking from the optical waveguide interfere with each other to generate interference light, and emitting the interference light from an end surface of the substrate, a photo-detector for monitoring the interference light of the radiation light and the signal light, and a signal controlling circuit for controlling an operating point of the optical modulator according to a change of the interference light monitored by the photo-detector It is thereby possible to prevent degradation of a distinction ratio due to a shift of the operating point of the optical modulator so as to allow stable optical modulation, while faci

Abstract: A device for stabilizing a modulation operation of an optical modulator is provided, the optical modulator having an optical waveguide provided on a substrate and an electrode provided so as to correspond to the optical waveguide, modulating light inputted to the optical waveguide by a voltage corresponding to a physical amount to be measured which is inputted to the electrode, and outputting the modulated light. The device includes: an optical detecting device for outputting signal corresponding to intensity of light outputted from the optical modulator; a control signal outputting device for, on the basis of the output signal of the optical detecting device, outputting a control signal which compensates a phase bias fluctuation of the optical modulator; a light source device for outputting control light corresponding to the control signal; and a voltage applying device, provided in contact with the electrode of the optical modulator, for applying voltage corresponding to the control light to the electrode.

Abstract: A viewing device that selectively attenuates light from bright objects, leaving light from dim objects substantially unaffected. The invention comprises an array of optical systems each of which contains one or more focusing elements which focus an image of the object field onto a focal plane, and one or more eyepiece elements for viewing portions of the focused image. The optical systems in the array are aligned to permit the viewed portions to be seen coherently to form a composite view of the object field. A photoactive layer is located at the focal plane. The photoactive layer may be a photochromic system that darkens when exposed to intense light and returns to the clear state when the intensity is substantially reduced. The light path through the device is folded using optical elements such as mirrors or prisms to minimize the size of the device. The photoactive layer may be a system of several layers functioning as a light valve.

Abstract: A phase modulator is provided in an externally modulated, feed-forward linearized, analog cable television (CATV) link for reducing fiber-induced composite second order (CSO) distortion components. The system is compatible with existing commercially available modulators. CSO distortion is caused, in part, by self-phase modulation of the transmitted signal in the fiber optic link. The problem is particularly prevalent when data is transmitted at a wavelength other than the minimum dispersion wavelength of the fiber. Moreover, CSO distortion increases with the length of the fiber optic link and with the optical intensity modulation index of the primary modulator of the link. The phase modulator modulates an optical signal which is provided to the primary modulator in accordance with an optical phase modulation index k of an RF data signal to be transmitted.

Abstract: An optical link is disclosed that is operated by a balanced detection technique so as to extend its linear dynamic range, yet substantially eliminate even-order harmonic distortion of the modulated signal. The modulator comprises two parallel arranged Mach-Zehnder interferometers that respectively have a steady state DC voltage bias applied so that the operating points of the Mach-Zehnder interferometers are selected to produce two separate modulated signals, one of which is later phase shifted by a phase shift device so as to provide an output that is combined with the other modulated signal in such a manner so as to cause a subtraction, leaving the final output signal substantially free of any unwanted even-order harmonic components in order to provide for a multi-octave response characteristic.

Abstract: An objective optical system for endoscopes comprising a plurality of lens components and an aperture stop, and configured so as to permit adjusting a distance to an object to be observed simply by exchanging a lens component disposed in the vicinity of the aperture stop with another lens component having a focal length different from that of the lens component disposed in the vicinity of the aperture stop. This objective optical system for endoscopes can have an optimum depth of field by changing a diameter of the aperture stop along with the exchange of the lens component disposed in the vicinity of the aperture stop.

Abstract: A device for viewing an object field containing intense light sources which could be disturbing or harmful to the human eye. Light from the object field is focused at a focal plane located within a layer of photochromic material. An intense light source produces an opaque mask in the photochromic layer. This mask matches the location and shape of the image of the intense light source at the image plane. An eyepiece permits a viewer to view the image plane to obtain an image of the object field with light from the intense light source dimmed by the opaque mask in the photochromic layer.

Abstract: A system for correcting nonlinear distortion in fiber optic systems including communication systems by means of light modulation provided by interferometric modulators such as the Mach-Zehnder. The present invention involves the addition of a correction signal which contains error canceling information. The additive correction signal optical beam is frequency shifted or phase scrambled through the use of a phase or frequency modulator. This insures the corrective light signal adds incoherently or quasi-incoherently within the signal bandwidth of interest. Furthermore, the present system requires only a single light source and can be made independent of wavelength and is easily optimized with active servo devices. A system provided by the present invention also provides improvement over the state of the art because of its high degree of correction and independence from link length using ordinary single mode fibers.

Abstract: An unbalanced Mach Zehnder optical frequency discriminator is linearized to suppress third order nonlinear distortion. An optical carrier that is modulated in optical frequency is input to an unbalanced Mach Zehnder interferometer to discriminate RF information signals carried on the carrier. The Mach Zehnder interferometer has an adjustable operating point. The operating point is adjusted to a point where the output intensity of the Mach Zehnder interferometer is no more than about ten percent of its maximum value. In order to avoid problems with second order distortion, the optical carrier is modulated with RF signals that do not occupy more than a single octave. The operating point of the device can be adjusted by adjusting an electrical bias thereof, by adjusting the wavelength of the optical carrier, or by adjusting the temperature of the device.

Abstract: A power beam tube is biased to supply a constant current to an EO crystal used as part of a frequency modulation circuit for FM laser radar. Integration of the constant current by the EO crystal's capacitance generates the desired ramp voltage waveform. A correction current is easily added to the circuit. The circuit effectively removes the EO crystal driver circuitry from the control loop, increasing the speed of the frequency modulator loop about ten times.

Abstract: An apparatus and method for image analysis utilizing an optical-electronic hybrid device. A light source generates light through an alternating series of lenses and electronically addressable spatial light modulators to an optical correlation plane. A first modulator introduces an input image and a second modulator introduces a Fourier transformed reference image into the device. The device correlates the input image and the reference image in the optical correlation plane. The optical correlation plane feeds back information through a processor to the second modulator which varies a parameter of the reference image. The second modulator varies parameters to determine the greatest correlation between the input image and the reference image. The parameters at the point of greatest correlation are used together with the reference image to identify the input image.

Abstract: An optical amplifying apparatus is disclosed which includes an optical amplifier for amplifying a signal light, a narrow-band optical filter for removing a spontaneous light emitted from the signal light, two photodetectors for detecting, transmitting and reflecting lights of the narrow-band optical filter, and a controller for controlling a gain of the optical amplifier by comparing intensities of the transmitting and reflecting lights in accordance with the detected signals supplied from the photodetectors.

Abstract: A control loop for matching the frequency of an incoming electromagnetic beam controls a local oscillator with two feedback loops including a coarse loop employing an optical frequency shifted beam to generate a coarse control voltage in nanoseconds and a fine loop employing a fast lock loop filter to generate a fine control signal that provides loop convergence on the order of a microsecond.

Abstract: An optical modulation source provides a pulsed optical signal 44 by coupling a pulsed optical pump signal 13 polarized along one axis of a polarization preserving fiber 24 having a wavelength .lambda.p and a continuous-wave (CW) probe signal 54 polarized partially along the pump polarization axis having the desired output wavelength .lambda.s provided by a laser 52, into one end 22 of a polarization preserving fiber 24. The pump signal is decoupled at the output end 26 of the fiber 24 and used to drive the laser 52 which provides the CW probe signal 54. The pulsed pump signal 13, when present, changes the polarization of the probe signal 54 by the optical Kerr effect. The probe signal is decoupled at the output end 26 of the fiber 24 and passed through an analyzer 38 yielding an output probe signal 44 which exists only when the pump signal is present (or alternatively, when absent), thereby providing a probe signal 44 having a wavelength .lambda.s of the probe signal and a pulse rate (1/T) of the pump signal.