A TERMINAL FOR A FREQUENCY DIVIDED, OPTICAL COMMUNICATION SYSTEM

Abstract: Optical terminal (1) has optical waveguides (6), 3-dB couplers (7), laser diodes (8, 20), laser diodes (22, 23) with gratings (21), and connections (24, 25) on substrate (5) of semiconductor material. The grating (21) laser diodes (22, 23) have external cavity formed by the waveguides (6) and delimited by mirrors (29). The laser diodes have each an electrical connection (30) through which the diodes are switched between light-absorbing, light amplifying and light-emitting modes, by electric current signals, and the diodes can also detect a light signal. The terminal (1) can perform several functions example: receive, detect, amplify and transmit a light signal. Generate a narrowband light signal of desired wavelength within a broad wavelength area through the grating diodes (22, 23) and amplify and transmit the signal. Receive a light signal having several wavelengths and to amplify and detect a selected one of the wavelengths through the cavity diodes (22, 23). Perform coherent reception of a light signal. Transmit a phase-modulated light signal. The terminal (1) comprises few types of components.

Description: A TERMINAL FOR A FREQUENCY DIVIDED, OPTICAL COMMUNICATION SYSTE TECHNICAL FIELD The present invention relates to a terminal for a frequency divided, optical communication system produced in integrate optics on a common semiconductor substrate having switchable lase diodes, optical waveguides with branches, mirrors and gratings the terminal including - at least two connections to the communication system; - a transmission waveguide which extends between the two connec tions and which is provided with at least one laser diode; - an extended optical cavity formed as a waveguide having at leas two reflecting end surfaces BACKGROUND ART A terminal in an optical communication network comprises plurality of known components, such as waveguide-optical direc tional couplers, photodetectors, waveguide branches and lase diodes. These components are described in detail in the literatu re. For example, an article in IEEE Spectrum, May 1984, pages 28 33, by S. Kobayashi and T. Kimura entitled "Semiconductor Optica Amplifiers" describes a semiconductor laserwhich is used both fo detecting and for amplifying optical signals. Coherent detectio of optical signals with the aid of a semiconductor laser i described in Electronics Letters 11th May 1989 Vol. 25 NO. 10 pages 680-682, J. Mellis and M.J. Creaner: "Coherent Detection o 565 Mbit/s DPSK Data Using Semiconductor Laser Amplifier as Phas Modulator". In an article in Electronics Letters 20th Novembe 1986, Vol. 22, No. 24, pages 1307-1308, by M.S. Whalen et a entitled "Wavelength-Tunable Single-Mode Fibre Grating Reflec tor", there is described a fan-shaped reflection grating which ca be adjusted to provide mutually different wavelengths. An optica directional coupler is described in US Patent Specification No. 4,157,860, and the US Patent No. 4,831,631 teaches a laser diod which includes an external resonance cavity. Waveguide branches, so-called passive 3-dB couplers are known, in which two incomin waveguides are joined to a commonwaveguide, which rebranches int two outgoing waveguides. A light wave in one of the incomin waveguides is divided into mutually equal powers in the two outgo ing waveguides. Waveguides in monocrystalline semiconducto materials are well known. In many cases, terminals in optical communication systems includ a plurality of the aforementioned components, and also other type of components. These components are discrete units which ar integratedto formrelativelycomplicatedterminals. Attemptshav been made to simplify these terminals, by producing integrate optical units on a common substrate. An example of one suc integrated unit is disclosed in an article in Appl. Phys. Lett. 5 (20), November 1987, pages 1577-1579, H. Inoue and S. Tsuj entitled "Optical amplification by monolithically integrate distributed-feedback lasers". The article describes an arrange ment including DFB-lasers, an optical switch, and amplifiers. A integrated heterodyne receiver is described in an articl presented in Electronics Letters 23rd November 1989, Vol. 25, No. 24, pages 1621-1623, by T.L. Koch et al and entitled "GalnAs/GalnAsP Multiple-Quantum -Well Integrated Heterodyne Receiver". This receiver includes a Bragg-reflector, a waveguide, a directional switch, amplifier and detectors. One problem with integrated units of this kind is that the various components differ greatly from one another with respect to their design, and the manufacture of the integrated unit requires a largenumber of separatemasks in separate production stages. This renders the units comparatively expensive and production yield is relatively low, in other words the number of useful units obtained is comparatively small. The Swedish Patent Application No.8803780-9 teaches a relatively simple terminal, which can be produced on one single substrate. The terminal is able to detect, amplify and transmit a light wave. One weakness of the terminal, however, is that it cannot be used in a broadband optical communication system that uses optical signals ofmultiple wavelengths. In a system of this kind, several wavelengths are handled simultaneouslyby a terminal, for instanc such that one of the wavelengths is detected and the remainin wavelengths are amplified and forwarded. DISCLOSURE OF THE INVENTION The aforesaid problems associated with a terminal for a broadban optical communication system are solved in accordance with th present invention with the aid of an optical integrated circui having but a few types of component on a common semiconducto substrate. The components primary used are semiconductor lase diodes, gratings for these diodes, and passive 3-dB couplers. Th components are mutually connected through waveguides and ar inventively arranged such as to enable a plurality of optica signals within a broad wavelength range to be processed simul taneously. Signal processing includes signal amplification direct and coherent detection, and transmission of a narrowban amplitude-modulated or phase-modulated light signal of desire wavelength within the whole of the broad wavelength range. The invention is characterized by the characterizing features se forth in the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS An exemplifying embodiment of the invention will now be describe in more detail with reference to the accompanying drawings, i which Figure 1 illustrates schematically a ring-shaped optical com munication system; Figure 2 illustrates an inventive terminal; Figure 3 is a block schematic which includes a laser diode an connected control means; Figures 4-8 inclusive illustrate different cases in the operatio of the terminal illustrated in Figure 2; and Figure 9 illustrates an alternative embodiment of the terminal. BEST MODE OF CARRYING OUT THE INVENTION A ring-shaped optical communication system is illustrate schematically in Figure 1. Terminals 1 are mutually connected b an optical fibre 2, and each of the terminals includes a respec tive control unit 3. These control units exchange electrica signals with the terminals 1, for the generation or registratio of optical signals P which are transferred between the terminal 1. The communication system is frequency divided, and a plura¬ lity of optical signals with light wavelengths λ ...λ.... wit narrow line widths are transferred on the optical fibre 2. Th signals together take up a relatively broad wavelength range According to one example, the light wavelengths lie in an regio of about 1.55 μm, the wavelengths λ„ are ten in number and eac wavelength channel has a line width of Δλ which corresponds to frequency in the order of 1-10 MHz. Optical isolator 4 can b placed between the terminals 1, so as to prevent optical signal being undesirably directed in the communication system. An exemplifying embodiment of the inventive terminals 1 i illustrated in Figure 2. A semiconductor substrate 5 supports optical waveguides 6, passive 3-dB couplers 7, laser diodes 8-20, a fan-shaped grating 21 and grating-provided, parallel-connected laser diodes 22 and 23. The waveguides 6 extend to the short sides of the substrate 5 and there form connections 24 and 25 for the optical fibres 2 in the communication system. The waveguide in which the laser diode 9 is located between the connections 24 and 25 forms a transmission waveguide for a light signal incoming to the connection 24 and delivered from the connection 25. The waveguides 6 have a further two branches which extend to the short sides of the base or substrate 5, and the waveguides have re¬ flecting end surfaces 27 and 28 adjacent these branches. The two connections 24 and 25 have low reflecting ability, and the end surfaces of said connections are obliquely positioned and coated with an antireflex coating 26. The two reflecting end surfaces 27 and 28 may be provided with mirrors 29, or each of said surf ces may be comprised of a respective cleaved crystal surface of the substrate 5, with a reflectance in the order of 30%. The laser diodes 8-20 and 22, 23 are travelling wave amplifiers of known construction, for example diodes according to the aforesai reference IEEE 1984 "Semiconductor optical amplifiers". The lase diodes have electrical connections 30 by means of which the diode can be controlled in the manner described in more detail herein after with reference to Figure 3. The fan-shaped grating 2 imparts individual grating constants to each of the diodes 22 an 23. These gratings form optical feedback elements which determin the fundamental values of the light wavelengths of the diodes These light wavelengths can be adjusted in the regions around th fundamental values, by applying electric current to the gratin parts of the laser diodes 22 and 23. This enables light signals t be produced with the narrow line width Δλ and with the centr frequencies λ. , which can be selected continuously within th aforesaid relatively broad wavelength range. The width of thi range can be selected by connecting in parallel a desired numbe of grating-provided laser diodes with the diodes 22 and 23. Th total wavelength range within which the light wavelengths can b generated at all, is determined by the semiconductor materials i the laser diodes and is of the order of about 50 nm. As mentioned in the introduction, one and the same laser diode i able to perform several different functions. This fact will b exemplified with reference to Figure 2. This figure illustratio includes, by way of example, the laser diode 9 whose electrica connection 30 is connected to an electric power source 51 and t a receiver 52. The power source supplies the laser diode 9 with current I whose value can be chosen within a relatively larg range. When the value I of the current is equal to zero, the diod will dampen an incoming light signal PI. In the case of a curren strength of up to a first threshold value, i.e. an amplifyin threshold, the diode will function as a light source. The diod becomes transparent to the incoming light signal PI at th amplification threshold. At elevated current strengths, up to second threshold value, a lasering threshold, the diode function as an amplifier with an amplification factor G, so as to obtain a outgoing light signal PU= GxPl. The diode 9 forms a laser when th current strength is further increased. When in its amplifying mod between the amplifying and lasering thresholds, the diode can als be used to detect the incoming light signal PI. A variation in th light strength of the signal causes a variation in the charg carrier concentration, which in turn results in a variation in th transition voltage of the diode 9. This voltage variation i detected by the receiver 52, which delivers a signal U correspon ding to the light signal PI. The diode 9 can now be used to detec the light signal PI, also when the current 1=0 and the diode i reverse biased electrically. The incoming light signal PI i herewith heavily absorbed and results in the formation o electron-hole-pairs in the diode. A voltage variation occurs across the diode, which can be detected by the receiver 52. In its reverse biased state, the diode functions as a very rapid light detector which is able to register significantly higher pulse frequencies than what the same diode is able to register in its amplifying mode. Examples of how the inventive terminal 1 can be used will be described below with reference to Figures 4-8. EXAMPLE 1. Amplifier and detector, Figure 4. The laser diodes 8, 9 and 10 are amplifying, as indicated with a symbol (+) at the connections 30. The laser diode 12 is reverse biased, as indicated by a symbol (-) , while remaining laser diodes are totally unbiassed. A light signal P2 of wave lengths λ -λ__ 1 N arrives at the connection 24, is amplified by the laser diodes 8, 9 and 10, and is delivered by the connection 25. Part of the light signal, which is branched upstream of the diode 12, reaches the laser diodes 17 and 18 and is there extinguished. If the light signal P2 contains only one of the wavelengths λ. , the pulses of the light signal can be detected in the reverse biased laser diode 12 up to a high pulse frequency. It is also possible in this case to detect the light signal P2 in one of the amplifying laser diodes 8, 9 or 10. EXAMPLE 2. Narrowband light source with broad tuning area, Figure 5. The laser diodes 10, 11, 13, 14, 15 and 16 are biased electricall in their forward directions and are amplifying, which is marked b the symbol (+) in the Figure, similar to the aforegoing. The lase diode 23 is also biased in its forward direction within it lasering range and the remaining laser diodes are totall unbiased, i.e.lack any form of bias. The laser diode 23 deliver light of the wavelength λ., the value of which is decided essen tially by the grating 21, as beforementioned. The laser diode 2 has an external cavity, which includes the waveguides 6 with th diodes 11, 13, 14 and 16 between the mirrors 29. Light from th laser diode 23 is reflected backwards and forwards between th mirrors in this cavity and is also amplified in the diodes 11, 13 14, and 16 besides being aplified in the diode 23. Part of th light energy is branched through a connecting waveguide with th laser diode 15, via the laser diode 10, and is delivered in th form of a narrow band light signal P3 by the connection 25. Par of the light energy in the cavity is reflected by the right han mirror 29 to the left in the Figure, and is branched off to th laser diodes 19 and 20 and to the connecting waveguide having th laser diode 12. The laser diodes 19, 20 and 12 extinguish th branched part of the light energy. The light wavelength can b selected, by supplying current to a selected one of the lase diodes 22 or 23. The light signal P3 can be modulated, for instanc by modulating the current I to the diode 10. EXAMPLE 3. Wavelength selective optical amplifier. Figure 6. The laser diodes 8, 10, 12 and 15 are electrically biased, so a to be transparent to a light signal P4 incoming to the connectio 24. The diodes 11, 13, 14 and 16 in the aforesaid external cavit between the mirrors 29 are forwardly biased electrically an provide a desired degree of amplification in the cavity. The lase diode 22 is absorbing, whereas the laser diode 23 is amplifyin and amplifies light of the wavelength λ. included in the incomin light signal P4. Only light which has the wavelength λ. i amplified in the cavity, because the diode 23 and its grating 2 are tuned around thiswavelength. Otherwavelengths, includingth wavelengths of light which is emitted spontaneously from th diodes, are dampened in the cavity. An amplified light signal P of wavelength λ. is delivered from the two connections 24 and of the terminal 1, but the rearwardlymoving light signal from t connection 24 is extinguished in an optical isolator, not show Two of the light wavelengths in the light signal P4 can amplified, by also biasing the laser diode 22 to its amplifyi mode. The light which is branched to the diodes 17-20 is exti guished in these diodes. EXAMPLE 4. Wavelength selective detector, Figure 6. In the Figure 6 illustration, the laser diode 9 is switched to it amplifying mode, the laser diode 15 is reverse biased, an remaining diodes are unchanged from Example 3. All wavelengths i the light signal P4 are amplified in the laser diodes 8, 9 and 1 and are delivered from the connection 25. Part of the light energ in the light signal P4 is branched to the cavity between th mirrors 29. As described above with reference to Example 3, desired wavelength of the wavelengths in P4 is amplified in th cavity. The light signal with this amplified wavelength i detected in the reverse biased laser diode 15. The detecte wavelength is selected by choosing which of the laser diodes 2 and 23 is amplifying. EXAMPLE 5. Coherent receiver, Figure 7. The laser diodes 8, 11, 13, 14 and 16 are forwardly biase electrically and are amplifying. The diode 23 in the externa cavity between the mirrors 29 is also forwardly biased an transmits light, and the diode 22 is absorbing. The two lase diodes 17 and 18 are reverse biased and function as light detec¬ tors. The diodes 12 and 15 are reverse biased so as to isolate th cavity effectively from the transmission line, whereas remaining laser diodes of the terminal 1 are absorbing. A coherent light signal P6 arrives at the connection 24, this signal having the wavelength λ. and carrying information in the form of a phase modulation. The diode 23 and its grating 21 function to generate a light signal of wavelength λ. in the cavity. Part of the light is branched from the cavity and absorbed in the diodes 19 and 20. Light having the well defined and constant wavelength . is branched from the cavity to the laser diodes 17 and 18 and interfers with the light signal P6. A difference signal, whi gives the phase information in the light signal P6, is detected the laser diodes 17 and 18. EXAMPLE 6. Phase modulator, Figure 8. The diodes 11, 13, 14 and 16 in the external cavity between t mirrors 29 are amplifying. The diode 22 is absorbing and the diod 23 with its grating 21 deliver light having the wavelength λ.. Th diode 12 is reverse biased so as to isolate the cavity from th connection 24. The current I to the diodes 10 or 15 is modulated as indicated in the Figure with a symbol (M) . Remaining diodes o the terminal 1 are absorbing. A light signal having the wel defined wavelength λ. is generated in the cavity and is branche to the connection 25. The branched light signal passes through th diodes 15 and 10 and is phase modulated in one of said two diode to a phase modulated coherent light signal P7. In an experimen carried out with the terminal 1, there was used in this modulatio process a modulation current of 10 mA, although theoretically current strength of 1 mA should be sufficient. The inventive terminal 1 can be used in more ways than are recite in Examples 1-6 above. For example, a light signal can be receive at the connection 24, amplified and transmitted through th connection 25 at the same time as the phase modulated light signa P7 is generated. In the illustrated exemplifying embodiment, th terminal is symmetric and can be used for light signals whic travel in both directions in the fibre 2 of the communicatio system illustrated in Figure 1. An alternative embodiment of the terminal will now be describe briefly with reference to Figure 9. This Figure illustrates terminal 31 on a substrate 38. Certain part of the terminal 31 hav the same design as in terminal 1. The transmission line betwee the connections 24 and 25 with the laser diode 9 is of the sam design as in terminal 1, as are also the coherent receivers wit the laser diodes 17-20. The terminal 31 differs from the termina 1 mainly in that laser diodes 33-36 are placed on one short sid of the substrate 38. The diodes 33-36 have a common fan-shape grating 37 and are connected with the remaining parts of th terminal througha tree-likebranching of optical waveguides. Eac of the laser diodes 33-36 has a mirror 39, which together with mirror 40 on the other short side of the substrate define a external cavity for each of the diodes. A laser diode 32 in waveguide which connects the two ends of the cavity forms a amplifier in said cavity. The embodiment illustrated in Figure has the advantage of being space saving, although it has th drawback, among other things, that a light wave which arrives a the cavity on the connection 24 unintentionally is able to reac the connection 25, without passing through the wavelength filtering grating 37. This results in undesirable transmission o allwavelengths inthis lightwave, althoughthe embodiment illus trated in Figure 2 effectively prevents such undesirable trans mission. The inventive terminals have the advantage of requiring only a fe number of component types, which simplifies production of th terminal. The terminal is able to perform a large number of th various functions required in a communication network, an separate functions can be achieved by switching currents. Optical signal processing in the terminal will not disturb signal trans¬ mission on the optical networktowhich the terminal is connected.

Claims: CLAIMS 1. A terminal (1) for a frequency-divided, optical communicatio system produced in integrated optics on a common semiconducto substrate (5) having switchable laser diodes (8-20) , optica waveguides (6) with branches (7) , mirrors (29) and gratings (21) , the terminal including - at least two connections (24, 25) to the communication system; - a transmission waveguide which extends between the two connec tions (24,25) and which is provided with at least one laser diod (9) ; - an extended optical cavity formed as a waveguide (6) having a least two reflecting (29) end surfaces (27, 28), said cavit having at least one laser diode (22, 23) provided with a wave¬ length grating (21) , the wavelength of said laser diode bein tunable around the grating wavelength (λ.), and at least on connecting waveguide provided with one of the laser diodes (12, 15) which connects the cavity with at least one of the connections (24,25) ; and - a coherent light detector for phase-correct detection of a light signal (P6) , said light detector being connected to the cavity and to one of the connections (24) , and said light detector having at least one laser diode (17, 18) for comparing the incoming light signal (P6) with a light wave of desired wavelength generated in the cavity, wherein the laser diodes (8-20, 22, 23) on the substrate (5) can be switched between their lightabsorbing, light-amplifying and light-transmitting modes with the aid of control signals (I) . 2. A terminal according to Claim 1, wherein the cavity has at least two laser diodes (22, 23) provided with wavelength gratings (21) , thewavelength gratings have different grating constants and the laser diodes (22, 23) are mutually connected in parallel. 3. A terminal according to Claim 1 or 2, wherein the cavity is connected to both the connections (24, 25) of the terminal (1) , by means of two separate connecting waveguides.

Inventors: GUSTAVSON MATS BOERJE, GUSTAVSON, MATS, BOERJE
Applicants: ERICSSON TELEFON AB L M, TELEFONAKTIEBOLAGET LM ERICSSON
Application year: 1992
Main classification: H04B10
IPC codes: G02B5, G02B6, G02F1, G02F2, H01L27, H01S3, H01S5, H04B10, H04J14