Optical interferometer and signal synthesizer using the interferometer

Abstract: A femtosecond pulse incident on a Mach-Zehnder interferometer is divided into two optical paths and thereafter synthesized again. The synthesized optical pulse is outputted as complementary two output optical pulses corresponding to the difference between optical paths. An optical interferometer for monitoring fluctuations of the optical path difference from one of two output optical pulses produced from an interferometer, effecting feedback on the optical path difference and providing an operation stabilized in a relation of 1/20 or less the central wavelength, and an optical signal synthesizer using the interferometer are provided.

Description: BACKGROUND OF THE INVENTION Field of the Invention: This invention relates to an optical interferometer and a signal synthesizer, and particularly to an optical interferometer applied to an optoelectronic device for performing signal processes for operations or communications, which are emploing optical pulses and a signal synthesizer using the optical interferometer. Description of the Related Art: On/off switching of output optical pulses using the difference in phase between two optical paths of a Mach-Zehnder interferometer is a well-known technique. However, since a long pulse of sub-nanoseconds or so has heretofore been used, the spectrum of the pulse is very narrow. Further, the conventional Mach-Zehnder interferometer controls the intensities of the output optical pulses but encounters difficulties in controlling their spectra. From GB-2 147 695 an optical interferometer is known with one output light path and with feedback light path length control. SUMMARY OF THE INVENTION The prior art has a problem in that although it performs intensity modulation on the output optical pulses with respect to relatively long pulse of sub-nanoseconds or so, the control of the spectra of the output optical pulses or the effective synthesis of signals by decompositing the spectrum and controlling each spectral component has not yet been taken into consideration. An object of the present invention is to provide an optical interferometer capable of allowing control of spectra, control of output optical pulses by utilizing a controlled stable interferometer and a femtosecond pulse having a wide bandwidth in combination, and a signal synthesizer using the optical interferometer, and a signal transmission method. Typical examples of various inventions of the present application have been shown in brief. However, the various inventions of the present application and specific configurations of these inventions will be understood from the following description. BRIEF DESCRIPTION OF THE DRAWINGS While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter which is regarded as the invention, it is believed that the invention, the objects and features of the invention and further objects, features and advantages thereof will be better understood from the following description taken in connection with the accompanying drawings in which: Fig. 1A is a block diagram showing one embodiment of a phase-locked interferometer according to the present invention; Fig. 1B is a diagram for describing a spectrum of an optical pulse inputted to the phase-locked interferometer shown in Fig. 1A; Fig. 1C is a diagram for describing a time characteristic of the input optical pulse; Figs. 2A through 2D are respectively diagrams illustrating examples of spectra of the optical pulse inputted to the phase-locked interferometer shown in Fig. 1A and spectra of output optical pulses; Fig. 3 is a diagram depicting an example of a configuration of a communication device according to one embodiment in which a phase-locked interferometer according to the present invention is used as a signal synthesizer; Fig. 4 is a diagram showing an example of a configuration of a communication device according to another embodiment in which a phase-locked interferometer according to the present invention is used as a signal synthesizer; Fig. 5 is a diagram illustrating a further embodiment in which a phase-locked interferometer according to the present invention is used as a signal synthesizer; Figs. 6A and 6B are respectively plan and side views showing a block diagram of an embodiment in which the signal synthesizer shown in Fig. 5 is formed on a semiconductor substrate; Figs. 7A and 7B are respectively cross-sectional views depicting examples of spatial light modulators applied to the signal synthesizer shown in Fig. 6; Fig. 8 is a diagram showing a modification of a part of the configuration of the signal synthesizer shown in Fig. 6; and Fig. 9 is a diagram illustrating an embodiment of another configuration of a two optical-path interferometer. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In order to achieve the above object, an optical path control means for stably controlling an optical path difference between divided optical pulses is provided in the present invention. Stable output optical pulses are obtained by holding the optical path difference in a predetermined relationship and respective spectra of the resultant output optical pulses are controlled by modulating signals, thereby making it possible to synthesize signals for optical communications. It is thus possible to perform multiplexed optical communications. A femtosecond pulse, which is incident to a Mach-Zehnder interferometer, is separated into two optical paths, followed by re-synthesis, which in turn is outputted as two complementary output pulses. The commonly used interferometer cannot obtain output pulses stabilized in the phase domain due to fluctuations of the optical path difference developed within the interferometer. As one method according to the present invention, a monochrometer is provided which monitors fluctuations of an optical path difference developed in the interferometer, based on one of two output optical pulses produced from the interferometer to thereby effect feedback on the optical path difference in the interferometer, thus providing stabilization in the phase domain. As another method according to the present invention, an interferometer is provided which makes up of a semiconductor and strict temperature control is performed to stabilize the fluctuations in the phase domain. If further necessary, then feedback for adjusting the length of the optical path is effected on one optical path to compensate for the fluctuations of the optical path difference as well as for the temperature control. When femtosecond pulses each having a wide spectrum are launched into the interferometer stabilized in the phased domain in this way, they interfere differently at each wavelengths. It is therefore possible to control the spectrum of each output optical pulse and bear or load a larger quantity of information on one pulse. According to the present invention, the two optical-path interferometer, for example, the Mach-Zehnder interferometer is capable of controlling the optical path difference in a resolution of 1/20 or less of the central wavelength of the input optical pulse, i.e., sub-femtoseconds if described by time. As a result, the spectrum of each pulse outputted from the interferometer with respect to the input femtosecond pulse can be stably controlled. Preferred embodiments of the present invention will hereinafter be described specifically with reference to the accompanying drawings. [EMBODIMENT I] Fig. 1A is a block diagram of one embodiment of a phase-locked interferometer according to the present invention. The present embodiment illustrates a phase-locked interferometer 50 which comprises a two optical-path interferometer 5 and a device for performing feedback to monitor fluctuations of an optical path difference developed in the interferometer and compensate for the fluctuations. The two optical-path interferometer 5 comprises two optical paths 7 and 7' formed by a beam splitter 4, mirrors 6 and a beam splitter 4. One optical path 7' is nothing but the configuration of a simple optical path. In the other optical path 7, the mirror 6 is mounted to a piezo electrically actuated optical stage 1 which vibrates with a frequency of a few thousand hertz and with an amplitude of about 1/40 the wavelength of an input optical pulse. Further, the piezo electrically actuated optical stage 1 is mounted to a piezo electrically actuated optical stage for compensation 2. The piezo-controlled optical stage 2 fine-adjusts the position of the mirror 6 according to a position control signal to be described later to thereby adjust or control its optical path length. Further, a motor controlled optical stage 13 for rough-adjusting the optical path length is interposed in the optical path 7. A predetermined interference status in the initial state of the interferometer can be created under the control of the motor controlled optical stage 13. Incidentally, the optical stages 1, 2 and 13 for controlling the optical path length are provided within the optical path 7 in Fig. 1A. It is however needless to say that the optical stages 1 and 2 may be provided within the optical path 7 and the optical stage 13 may be provided within the optical path 7' and that the optical stages 1 and 2 can be in the optical paths 7' and 7', respectively. As an optical pulse source inputted to the two optical-path interferometer 5, a femtosecond titanium sapphire laser pumped by an argon laser was used. The width of a pulse outputted from the laser is 10fs. The present laser pulse is allowed to pass through a β - B a B 2 O 4 (BBO) crystal to thereby generate second harmonics (whose wavelength ranges from 370nm to 410nm) (pulse width ranging from 12fs to 15fs). Thereafter, a pulse 3 compressed through a prism pair was obtained. Examples of a spectrum characteristic of the input pulse or optical pulse 3 and a time characteristic thereof are illustrated in Figs. 1B and 1C respectively. When the input optical pulse 3 is launched into the Mach-Zehnder interferometer 5 having the beam splitters 4 and the mirrors 6, interference occurs according to the difference between the optical paths in the interferometer 5, so that output optical pulses having complementary spectra λ 1 and λ 2 can be obtained from the optical paths 7 and 7'. The phase of output of the optical paths 7 and 7' differs by π (1/2 of an optical cycle). The output optical pulse λ 2 obtained from one 7' of the two optical paths 7 and 7' in the interferometer is allowed to pass through a spectroscope (e.g., diffraction grating) 8 and the intensity of a predetermined wavelength within the spectrum Fig.1B is measured by an photo-detector 9. When the difference between the optical path lengths of the interferometer 5 fluctuates, the intensity of the output produced from the spectroscope 8 varies and hence the output of the photo-detector 9 varies. It is thus possible to monitor fluctuations of the optical path difference of the interferometer 5. The piezo-controlled optical stage 1 is vibrated by an oscillator 12 and it is possible to obtain a component synchronized with the vibration of the optical intensity output of the photo-detector 9 from a lock-in amplifier 10. The lock-in amplifier 10 detects a variation in the optical path difference from a variation in the output of the photo-detector 9 and compensates for the detected variation, and outputs a compensation signal 11 to the optical stage 2 so as to hold the optical path difference constant. It was thus possible to control the interferometer so as to always hold the spectrum of the optical pulse 7 outputted from the interferometer constant. Since the piezo-controlled optical stage 1 is vibrated by the oscillator 12, the present embodiment can offer high sensitivity to the change in the optical path difference. Although omitted in the aforementioned description, reference numeral 14 in Fig. 1A indicates one example of a portion for inputting a modulating signal. This portion corresponds to an addition circuit which adds a signal S to the output of the lock-in amplifier 10. Thus, when the signal S is added to the output of the lock-in amplifier 10, an error would be as if to have occurred in the optical path difference by its addition. For example, the difference between the two optical paths 7 and 7' is reversed so that the output optical pulses of spectra λ 2 and λ 1 are obtained from the optical paths 7 and 7' respectively. Namely, the addition circuit results in one necessary when the interferometer is used as a signal synthesizer by controlling the optical path difference of the interferometer 5 shown as the embodiment. This will be described later. Figs. 2A through 2D are respectively diagrams illustrating examples of spectra of the optical pulses inputted to the phase-locked interferometer 50 shown in Fig. 1A and spectra of output optical pulses. Referring to Fig. 2A, a thick solid line 3 indicates the spectrum of an incident pulse. Symbols λ 1 and λ 2 respectively indicate output spectra at the time that the phase of the optical path difference of the interferometer is 2nπ and 2nπ + π (where n is an arbitrary integer but indicates n = 36 in Fig. 2A) where the frequency detected by the spectroscope 8 is slightly shifted from the central frequency (7.544 x 10 14 Hz) of the incident pulse. Referring to Fig. 2B, a thick sold line 3 indicates the spectrum of an incident pulse. Symbols λ 1 and λ 2 respectively indicate output spectra at the time that the phase of the optical path difference of the interferometer is 2nπ and 2nπ + π (where n = 36) where the detected frequency of the spectroscope 8 is set to the central frequency (7.544 x 10 14 Hz) of the incident pulse. Even if the phase of the optical path difference of the interferometer is set to 2nπ + π/2 and 2nπ - π/2 (where n = 36) where the detected frequency of the spectroscope 8 is set to the central frequency (7.544 x 10 14 Hz) of the incident pulse, output spectra similar to those shown in Fig. 2A can be obtained. Figs. 2C and 2D respectively show the states of two pulses in the interferometer 5 at the time that the difference in time between the two pulses is rendered larger (greater than n=36). The number of peaks of the frequency in the output spectrum can be arbitrarily set by setting n on an arbitrary basis. [EMBODIMENT II] Fig. 3 is a block diagram showing a signal synthesizer for controlling pulses λ 1 and λ 2 outputted from a phase-locked interferometer 50 according to 1/0 of a modulating signal S and an example in which communications are made by the signal synthesizer. As described in relation to Fig. 1A, when a voltage having a suitable magnitude is applied to the addition circuit 14 in response to 1/0 of the modulating signal S, the error would be as if to have occurred in the optical path difference correspondingly. For example, the difference between the two optical-paths 7 an 7' is reversed so that output optical pulses having spectra λ 1 and λ 2 can be obtained from their corresponding optical paths 7 and 7'. Namely, the phase-locked interferometer can be constructed as the signal synthesizer by controlling the optical path difference of the interferometer 5. An incident pulse 3 is supplied to the phase-locked interferometer 50 and a signal corresponding to a signal to be transmitted is applied to the phase-locked interferometer 50. The spectrum of the output optical pulse produced from the interferometer 50 according to the signal S results in λ 1 or λ 2 . Namely, it is possible to generate a signal corresponding to an output optical pulse of a spectrum λ 1 or λ 2 corresponding to 0 or 1 of the signal S. In this case, either one of the optical paths 7 and 7' may be used. This signal pulse is allowed to pass through an optical transmission or transfer medium 19 so as to be transmitted to signal receiving points. Signals at receiving points are detected as follows: Optical pulses 21 diffracted to different positions according to the wavelength by a diffraction grating 20 are detected by their corresponding photo-detectors 22 to thereby make it possible to determine the signal of 0 or 1. Although the atmosphere can be used as the optical transfer medium 19, a wave-guiding channel or waveguide, or optical fiber may be used. [EMBODIMENT III] In the present embodiment, an output spectrum obtained from an optical path 7 or 7' of a phase-locked interferometer 50 is set so as to have a number of central frequencies as shown in Figs. 2C and 2D and it is controlled so as to be held constant. The output optical pulse obtained from either one of the optical paths 7 and 7' is spatially separated into parallel beams by prisms 61 and 62. Thereafter, an absorption light modulator 63 is placed on optical paths of the parallel beams. A one-dimensionally space-dependent light modulator using a liquid crystal, which will be described later as a specific example, was used as the light modulator 63. The light modulator 63 is supplied with a signal S to control the turning on and off of frequencies corresponding to respective locations or places for the light modulator 63. After the parallel beams are gathered again by prisms 64 and 65, they are allowed to pass through an optical transfer medium 19. Here, lenses may be used in place of the prisms 62 and 64. After the respective wavelengths are spatially separated by a prism 66, the transmitted signal is detected by a position-dependent one-dimensional photo-detector 67. Namely, position information about the turning on and off of the optical pulses at the optical modulator 63 can be transmitted to the photo-detector 67. The present embodiment has described the case in which either one of the optical paths 7 and 7' is used. However, if similar devices are provided for the output optical pulses obtained from the respective optical paths, then the signal can be transmitted twice. Since the signal can be separated on the receiving side every wavelengths in this case, the transmission itself may be integrated into one transmission path. Namely, according to the present embodiment, the electric signal for controlling the light modulator 63 is converted into the on/off of light intensity to thereby make it possible to transmit a signal in wavelength multiple form. (EMBODIMENT IV] In the present embodiment, a phase-locked interferometer will be explained, which makes use of both the aforementioned optical paths 7 and 7' and mounts them on a semiconductor substrate. Fig. 5 is a block diagram showing a basic configuration of the phase-locked interferometer and draws even signal waveforms at respective input/output positions side by side together with the configuration of the phase-locked interferometer. Reference numeral 100 indicates a Mach-Zehnder interferometer of two optical paths, into which an optical pulse 3 having the continuos spectrum and time characteristics shown in Figs. 1B and 1C is launched. The output of the interferometer 100 is divided into two whose respective spectra result in discrete spectra like λ 1 and λ 2 . Now consider that the respective peaks of the spectrum λ 1 fall within frequencies f 1 , f 3 , f 5 , ... , respectively whereas the respective peaks of the spectrum λ 2 fall within frequencies f 2 , f 4 , f 6 , ... , respectively. Reference numerals 200 and 300 indicate multichannel modulators into which optical pulses having spectra λ 1 and λ 2 are launched, respectively. Each channel of the multichannel modulator 200 and 300 corresponds to the modulation channel of a specific frequency. The multichannel modulators 200 and 300 are respectively supplied with modulating signals S 1 and S 2 and modulate the input optical pulses in response to these signals. As a result, the modulated optical pulses result in optical signals having spectra designated at numerals 201 and 301 respectively. Reference numeral 400 indicates an optical coupler which makes use of a half mirror, for example. The coupler 400 re-synthesizes the optical signals having the spectra 201 and 301 and produces the final output 401 therefrom. The optical spectrum of the final output 401 lacks partly, as shown in the drawing, in association with the modulating signals S 1 and S 2 . Output pulse waveform 402 indicating time characteristic greatly varies from the original input pulse waveform 3. An arbitrary pulse waveform is generated according to the modulating signals inputted to the multichannel modulators 200 and 300. The final output 401 results from the synthesis of the optical pulses having the spectra 201 and 301. Either of the two outputs from the optical coupler 400 may be used. Figs. 6A and 6B show a specific embodiment mounted on a semiconductor substrate. The present embodiment shows a signal synthesizer implemented by optical transparent triple layers provided on the semiconductor substrate. Reference numeral 2000 indicates a semiconductor substrate. Transparent triple layers 1003, 1002 and 1001 are stacked on the semiconductor substrate 2000. Refractive indexes of these triple layers are respectively defined as n 3 , n 2 and n 1 for the layer 1003 on the substrate side, the layer 1002 formed as the intermediate layer and the layer 1001 formed as the outermost layer where n 1 < n 2 and n 3 < n 2 are satisfied. Light to be introduced from the outside is focused into the intermediate layer 1002 and allowed to propagate with the light trapped in this layer. The triple layers used herein can be formed as a polymer film, for example, and in addition, it may be formed by a semiconductor layer. An example, in which an optical element is constructed of such triple layers, has been introduced in Fig. 9 entitled Optical Micromachine in OYOBUTURI (The Japane Society of Applied Physics), Vol. 66, No. 1, pp. 9 - 14. Reference numeral 3000 indicates a constant temperature device. The semiconductor substrate 2000 is placed on the constant temperature device 3000. Namely, the signal synthesizer mounted on the upper surface of the semiconductor substrate 2000 is maintained at a constant temperature in the present embodiment. The incident light or optical pulse having the spectrum shown in Fig. 1B is introduced into a waveguide 101 formed by the transparent triple layers 1001, 1002 and 1003. The part except waveguide structure has no triple layers. The incident light is introduced into an optical path constituting the interferometer 100 through the waveguide 101. Since the other waveguide 102 contacts the waveguide 101 so as to form a separation path in the interferometer 100, the incident light is divided into two optical-paths corresponding to the waveguides 101 and 102. These two optical-paths are brought into contact with each other again at positions where their optical path lengths differ from each other. As a result, interference occurs in the above-described incident light and hence waveforms of spectra λ 1 and λ 2 shown in Fig. 5 are obtained from exits of the optical paths 101 and 102. The peak-to-peak frequency-difference of each waveform can be determined depending on the adjustment of each optical path length as described above. If further required, then a refractive index controller designated at numeral 110 is inserted into the optical path 101. The refractive index controller 110 can fine-adjust equivalently the substantial length of the optical path 101. The refractive index controller 110 is controlled by the voltage applied to each electrode provided at an upper portion of the optical path 101 to thereby control the refractive index. If required to stable the interferometer 100 in an analogy of Fig.1A, then an optical path 106 is used as a monitor, corresponding to the optical path 7'. The compensation signal 11, in this case, is acted to the refractive index controller 110 or the temperature controller. The light emitted from the optical path 101 and the light emitted from the optical path 102 are introduced into the multichannel modulators 200 and 300, respectively. In the multichannel modulators 200 and 300, regions indicated by dashed lines in the drawing are all built of triply piled-up layers. Further, the multichannel modulators 200 and 300 are substantially identical in configuration to each other. The light or optical pulses launched from the optical paths 101 and 102 to the multichannel modulators 200 and 300 are set to parallel light beams by cut-away portions 210 and 310 having plano-concave shapes acting plano-convex lenses, respectively. The parallel light beams transmitted through the cut-away portions 210 and 310 are respectively introduced to cut-away portions 211 and 311 serving as diffraction gratings. The light or optical pulses separated into every frequency by the diffraction gratings 211 and 311 are focused on the spatial light modulators 231 and 331 every frequency through cut-away portions 221 an 321 acting plano-convex lenses. As described in Figs. 7A and 7B showing portions of the spatial light modulators 231 and 331 in the form of cross sections, the spatial light modulators 231 and 331 allow the light or optical pulses subjected to optical modulation every frequency to pass therethrough because voltages for varying absorptance or refractive index are respectively applied to focusing positions associated with each frequency. The optical pulses transmitted through the spatial light modulators 231 and 331 are respectively introduced to cut-away portions 251 and 351 used to serve as diffraction gratings through cut-away portions 241 and 341 functioning as plano-convex lenses, where they are restored to the parallel light beams. The optical pulses restored to the parallel light beams are focused on their corresponding optical paths 103 and 104 through cut-away portions 261 and 361 acting plano-convex lenses. The optical pulses introduced into the optical paths 103 and 104 are coupled by the optical coupler 400 so that a signal waveform like the spectrum 401 shown in Fig. 5 is obtained. Figs. 7A and 7B are respectively cross-sectional views of the portions of the spatial light modulators 231 and 331 as viewed at positions taken along lines A - A in Fig. 6A. Reference numeral 2000 indicates a semiconductor substrate. Reference numerals 1003, 1002 and 1001 respectively indicate transparent layers stacked in three layers. Their refractive indexes are defined as n 3 , n 2 and n 1 (where n 1 < n 2 and n 3 < n 2 ). Reference numerals 85 respectively indicate electrodes which are provided on the transparent layer 1001 corresponding to the uppermost layer, where the electrode 85 and the substrate 2000 are electrically insulated. The light transmitted through the spatial light modulator 231 has only frequency components of f = f 1 , f 3 , f 5 , ... as in the spectrum λ 1 . The positions of the electrodes, which are necessary for modulation, are represented as shown in Fig. 7A. Similarly, the light transmitted through the spatial light modulator 331 has only frequency components of f = f 2 , f 4 , f 6 , ... as in the case of the spectrum λ 2 . The positions of the electrodes, which are necessary for modulation, are represented as shown in Fig. 7B. In Figs. 7A and 7B, different hatchings are provided in alternate order to clearly express the frequencies of the light transmitted through the intermediate transparent layer 1002. If the voltages applied (applied between the electrodes 85 and the semiconductor substrate 2000) to the electrodes 85 of the spatial light modulators 231 and 331 are controlled according to the modulating signals S 1 and S 2 , then arbitrary modulation can be done. In the embodiment shown in Fig. 6A, the all transparent layers can be cut out in plano-concave shapes form to form the plano-convex lenses and their stepwise cutting can form the diffraction gratings. Therefore, the whole can be configured on the semiconductor substrate. In the present embodiment, the temperature control of the semiconductor substrate with high accuracy allows the implementation of a controlled stable interferometer. Thus, according to the present embodiment, the control system (for monitoring the fluctuations of the optical path difference of the interferometer according to the output optical pulse and thereby performing feedback on the optical path difference in the interferometer) described in Fig. 1A may be omitted. Of course, feedback, compensating the fluctuations of the optical path difference and controlling the voltage applied to the refractive index controller 110 shown in Fig. 6A, can be also made as well as the strict temperature-control of the semiconductor substrate. A spectroscopic function, comprised of the cut-away portions 210 and 310 serving as the plano-convex lenses, the cut-away portions 211 and 311 serving as the diffraction gratings and the cut-away portions 221 and 321 serving as the plano-convex lenses employed in the embodiment shown in Fig. 6A, can be also performed using a waveguide grating shown in Fig. 8. The waveguide grating is disclosed in Fig. 2 of the paper "Multifrequency Laser for Dense WDM Applications" introduced in the European Conference on Integrated Optics'97 subsequent to the filing of the present application. Light is incident or inputted from the left end in the drawing and is transmitted successively through a free space region, a waveguide, and another free space region. Then light is introduced to each waveguide associated with each frequency, and finally amplified through an embedded layer. It can be easily understood that if this is utilized in reverse, then a light-gathering function, comprised of the cut-away portions 241 and 341 serving as the plano-convex lenses, the cut-away portions 251 and 351 serving as the diffraction gratings, and the cut-away portions 261 and 361 serving as the plano-convex lenses, can be achieved in the same manner as described above. Fig. 9 is a diagram showing an embodiment of another configuration of the two optical-path interferometer. The present embodiment is different from the embodiment of the interferometer 100 shown in Fig. 6A, because the length of two optical-paths are substantially formed equally, and the refractive index of one of those two optical-paths, is changed so that the optical paths are equivalently different in length from each other. In the drawing, reference numerals 101 and 102 respectively indicate optical paths similar to those illustrated in the embodiment of Fig. 6A. Reference numeral 105 indicates an electrode. The refractive index can be controlled by controlling the voltage (applied between the electrode 105 and the semiconductor substrate 2000) applied to the electrode 105. As a result, the optical path lengths can be equivalently rendered different from each other even if the optical paths are structurally identical in length to each other. The present embodiment has the merit of being capable of accommodating fluctuations in manufacture by control. According to the present invention, a stable interferometer can be obtained in a resolution of 1/20 or less of the central wavelength of an input optical pulse. Further, the interferometer of the present invention can control the spectra of output pulses obtained when femtosecond pulses are inputted. Further, since the output of the interferometer can be brought to discrete spectrums, light can be adjusted so as not to exist in a light-unmodulable region on a spatial light modulator. If both optical paths of the interferometer are used for signal synthesis, then optical synthesizing can be totally effected on a continuous spectrum over the entire range of the spectrum. While the present invention has been described with reference to the illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to those skilled in the art on reference to this description. The invention is defined by the appended claims.

Claims: An optical interferometer (50) comprising: a two optical-path interferometer (5) having means (1, 2) for controlling the length of one of the two optical paths in the interferometer, and arranged with two optical paths (7,7') for outgoing light of said interferometer, means (8, 9) for detecting fluctuations in the difference between optical path lengths in said interferometer, said means (8,9) arranged in one of the two optical paths (7, 7') for outgoing light of said two optical-path interferometer (5), and means (10) for adjusting the difference between the optical paths to compensate for the detected fluctuations so that an operation of the interferometer is stabilized. The interferometer of claim 1, wherein the adjusting means (10) include means for automatically feeding back a signal (S) corresponding to the fluctuations to said control means. The optical interferometer of claim 1 or 2 wherein said detecting means include means (8) for spectroscopically separating an outgoing optical pulse obtained from one of the two optical paths for outgoing light into its spectral optical pulses, and means for detecting variations in the intensity of each separated optical pulse. The interferometer of claim 3, wherein said detecting means further include detecting means (9) for detecting the intensity of a predetermined wavelength component of each separated optical pulse. The interferometer of any of the preceding claims, wherein said adjusting means further include means for supplying a signal equivalent to the result of the variation in the difference between the optical paths to said optical-path difference adjusting means in response to signals to be synthesized. The interferometer of any of the preceding claims, wherein the two optical path interferometer is formed on a semiconductor substrate, and comprises a first optical path (101) for receiving incident light and a second optical path (102) separated from said first optical path after contacting the first optical path once for splitting the incident light and brought into contact with said first optical path again so that their optical lengths differ from each other, wherein the optical paths (101, 102) are formed by a three stacked layers (1001, 1002, 1003) whose interposed one (1002) has a higher refractive index than the other two layers (1001, 1003), and the adjusting means (110) are coupled to said semiconductor substrate. The interferometer of any of the preceding claims, wherein the difference between said optical paths is achieved by varying the optical path lengths or refractive indexes. The interferometer of any of claims 1 to 5, wherein said two optical-path interferometer comprises optical stages and said optical-path length control means is driven by a piezoelectric actuator. The interferometer of claim 6, wherein said optical path difference adjusting means is arranged to control the voltage applied to one of the optical paths or to control the temperature of a part of one of the optical paths to vary the refractive index of the optical path. The interferometer of any of the preceding claims, arranged so that in use the phase of the difference between the optical path lengths is arbitrarily set. The interferometer of any preceding claim, wherein, when in use, the optical pulse applied to said two optical-path interferometer is a femtosecond pulse. An optical signal synthesizer with an interferometer of any of claims 3 to 5, further comprising means for converting said separated optical pulse into parallel light beams, arranged so that in use the optical paths of respective wavelengths of said parallel light are turned on and off according to intended signals. The synthesizer of claim 12, further including means for independently turning on and off the optical paths of the respective wavelengths of the parallel beams, which are independently obtained from the outgoing optical pulse obtained from the two optical-paths for the outgoing light of said interferometer, and means for synthesizing the turned-on/off parallel beams into one optical path and combining the two optical-paths into one optical path. An optical synthesizer with an interferometer of any of claims 6, 7 or 9, wherein the interferometer is arranged to separate incident light into two light beams, further comprising means for decomposing each separated light beam according to frequencies, modulation means for subjecting the resulting decomposed light to modulation by spatial light modulators according to each frequency, and re-synthesizing means for re-synthesizing an output of said modulating means, and means for coupling the re-synthesized beams, whereby said respective means are provided on a semiconductor substrate. The synthesizer of claim 14, wherein, the modulator means and the re-synthesizing means are multi-channel modulators formed on said semiconductor substrate, and the coupling means is an optical coupler formed on said semiconductor substrate for the synthesizing light of third and fourth optical paths for receiving the re-synthesized respective beams. The synthesizer of claim 15, wherein, each multi-channel modulator provided on the semiconductor substrate is formed by three stacked layers, whose interposed one has a higher refractive index than the other two layers and is constructed such that a cut-away portion serving as a convex lens, a cut-away portion serving as a diffraction grating, a cut-away portion serving as a convex lens, a spatial light modulator for varying the refractive index at a focusing position corresponding to the frequency according to a modulating signal, a cut-away portion serving as a convex lens, a cut-away portion serving as a diffraction grating, and a cut-away portion serving as a convex lens are arranged in cascade form. A signal transmission method comprising the steps of: applying a femtosecond optical pulse to one of the optical paths of a two optical-path interferometer, stabilizing the difference between the optical paths of said interferometer by detecting an optical pulse obtained from one of the two optical paths for outgoing light of said interferometer, controlling the spectrum of each optical pulse of the outgoing light of said interferometer according to a signal to be transmitted, transmitting the controlled optical pulses, and producing the transmitted signal by optically coupling the transmitted optical pulses. The method of claim 17, wherein said stabilizing step is executed by detecting variations in the intensity of a predetermined frequency component of the outgoing optical pulse obtained from said one optical path, thereby adjusting the optical path difference. The method of claim 17 or 18, wherein said interferometer is formed on a semiconductor substrate and said stabilizing step is performed by controlling the temperature of the semiconductor substrate.

Inventors: HEBERLE ALBERT, NAGANO HISASHI, OGAWA SUSUMU, PETEK HRVOJE, TOMARU TATSUYA, HEBERLE, ALBERT, NAGANO, HISASHI, OGAWA, SUSUMU, PETEK, HRVOJE, TOMARU, TATSUYA
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1997
Main classification: G01B9
IPC codes: G01J9