Improvements in and relating to semiconductor lasers

Abstract: A method and apparatus is disclosed of generating controlled light signals (8) in a laser transmitter (1) of the type having a stimulated emission cavity (4) and at least two cavity sections. This is achieved by control of input current signals according to pre-defined characteristics for light power and wavelength. Examples of the applications are in time division multiplexing and transcoding.

Description: The invention relates to a method of operating a semiconductor laser transmitter of the type comprising a stimulated emission cavity and at least two separate contacts, each of which contact is associated with a region of the cavity. The method comprises the steps of injecting electrical signals to the contacts to generate optic signals. The invention also relates to a semi-conductor laser transmitter assembly for carrying out the method. Heretofore, it has been known that light is emitted from such a semiconductor laser transmitter according to a complicated relationship between carrier and photon concentrations of adjacent cavity sections. This relationship has not, however, been understood and consequently such laser transmitters have had few practical applications. The prior art Journal of Lightwave Tech., vol.LT5 no. 4, April 1987, Pages 516-522 IEEE, Y. Yoshikuni et al. refers to operation of lasers with multi-electrodes. In this document, relations between parameters of output light signals and the injected electrical signals are not defined in a manner which allows control of the transmitter to achieve practical uses. European Patent Specification Nos. 173269, 128297, and 189252 (NEC Corp.) also relate to control of laser transmitters having a plurality of contacts. Currents are injected to these contacts so that there is an active waveguide section and a control waveguide section to achieve frequency stabilised and frequency modulated outputs. The invention provides a method of operating a semiconductor laser transmitter of the type comprising a stimulated emission cavity and at least two separate contacts, each of which contact is associated with a region of the stimulated emission cavity, the method comprising the steps of injecting electrical signals to the contacts to generate optic signals characterised in that, the electrical signals are electronic data signals having a magnitude sufficient to drive each associated cavity region above a stimulated emission threshold level to generate cross-coupled stimulated emission by interaction of photons of each region with carriers of the or each other region; and the electronic data signals are varied to control optic intensity and wavelength according to pre-defined characteristics representing response of optic intensity and wavelength to changes in cavity region carrier concentrations so that the laser transmitter is operated as an opto-electronic data processor for processing the electronic data signals to generate optic data signals, the pre-defined characteristics being representable by constant wavelength and constant intensity contours as a function of carrier concentrations in the cavity regions. By injecting electrical signals in this manner opto-electronic data processing is achieved with accurate control of both wavelength and intensity of the output light. In one embodiment, the laser transmitter is operated as a time division multiplexer for data channels delivered to each contact and synchronised with clock signals also delivered to each contact at a rate approximately double that of each data channel, said clock signals driving the transmitter between two carrier concentration states, at each of which state a substantial optic intensity excursion will occur for electronic data input at one contact only. In this way, combined electronic to optic conversion and time division multiplexing is achieved in a simple manner. In another related embodiment, the semiconductor laser transmitter is operated as a combined time division and wavelength multiplexer for said data channels and an additional data channel delivered to a contact by varying the clock signals delivered to each contact so that the optic data signals have a different wavelength at each operating state. This allows for more versatility and usefulness of the transmitter. In another embodiment, two data channels may be delivered to each contact for four-to-one multiplexing. In a still further embodiment, the laser transmitter may be operated as a pulse duration to pulse position transcoder by varying the electronic data signals to two of the contacts in a mutually inverse relationship to generate an optic intensity excursion of a complete cycle to form a pulse for each electronic data level change. This is an example of another type of data processing which may be achieved by appropriate control of the laser transmitter. In this latter embodiment the electronic data signals may be derived from a single data signal delivered directly to one contact and inverted before delivery to the other contact. This is a simple and inexpensive manner of operation of the transmitter. According to another aspect, the invention provides a semiconductor laser transmitter assembly comprising:- a semiconductor laser transmitter comprising a stimulated emission cavity and having at least two separate contacts, each of which is associated with a region of the stimulated emission cavity; characterised by means for injecting electronic data signals to the contacts to drive each associated cavity region above a stimulated emission threshold level thereof to generate cross-coupled stimulated emission by interaction of photons of each region with carriers of the or each other region; and means for varying the electronic data signals to control optic intensity and frequency according to pre-defined characteristics representing response of optic intensity and frequency to changes in cavity region carrier concentrations so that the laser transmitter is operated as an opto-electronic data processor for processing the electronic data signals to generate optic data signals, the pre-defined characteristics being representable by constant wavelength and constant intensity contours in response to the carrier concentration. The invention will be more clearly understood from the following description of some preferred embodiments thereof, given by way of example only with reference to the accompanying drawings in which:- Figure 1 is a diagrammatic perspective view from the above of a conventional semiconductor laser transmitter; Figs. 2 to 4 are diagrams illustrating operation of the semiconductor laser transmitter of Fig. 1; Fig. 5 is a graph illustrating operating characteristics of the laser transmitter; Fig. 6 is a circuit diagram of a semiconductor laser transmitter and a signal input circuit according to the present invention; Fig. 7 is a graph illustrating operating characteristics of the transmitter of Fig. 6; Fig. 8 is a circuit diagram of a laser transmitter and an alternative construction of signal input circuit according to the present invention; Fig. 9 is a graph illustrating operating characteristics of the laser transmitter of Fig. 8; and Fig. 10 is a set of mathematical expressions characterising operation of a semiconductor laser. Referring to the drawings, and initially to Fig. 1, there is illustrated a conventional semiconductor laser transmitter indicated generally by the reference numeral 1. The laser transmitter 1 comprises a p-n junction diode 2 with an active layer 3, and a cavity 4 in which light stimulated emission takes place. The laser transmitter 1 also includes input contact means, namely, two drive contacts 5a and 5b having signal input conductors 6a and 6b, respectively, connected to the diode 2 at an upper side of the cavity 4. The laser transmitter 1 also includes a lower contact 7. A laser beam emitted light output from the cavity 4 is indicated by the lines 8. The semiconductor laser transmitter 1 is conventional and has been described merely to help in understanding the invention. To assist in an understanding of the invention the following is a brief description of the manner in which we have gained an understanding of the operating characteristics of such a semiconductor laser. Referring now to Figs. 2 to 4, it will be seen that in use, input signals, namely, drive current signals I₁ and I₂ are supplied to the drive contacts 5a and 5b respectively. These drive current signals vary the photon ( γ) and carrier (N) concentrations of the cavity adjacent the drive contacts 5a and 5b. The separate drive contacts 5a and 5b, therefore, create two separate cavity sections 10 and 11 respectively. In these drawings, end facets of the laser transmitter 1 are indicated by the numerals 12 and 13 and the symbol "U" denotes equivalent input flux to the facets. To model operation of the laser transmitter 1, the Fabry-Perot approach has been found to be suitable. This approach recognises the laser transmitter as a resonator containing two active amplifying cavity sections. This model also allows wavelength of emitted light to be calculated according to the relationship (I) of Fig. 10, in which N is the carrier concentration, and n is the refractive index. This model also allows for the fact that the drive current signals may be controlled independently while at the same time the carrier concentrations N₁ and N₂ are coupled by the shared photon field via the stimulated emission process. Spontaneous emission is represented by the equivalent input fluxes U₁ and U₂ to each facet. In Fig. 4, A and B represent the amplitudes of two counter-propagating photon fluxes. Using expressions for photon density as a function of the travelling fluxes and end facet boundary conditions, expressions for the amplitude of the travelling waves A and B as a function of position in the cavity 3 may be obtained. These expressions lead to an analytical expression of the mode intensities as given in expressions (II) and (III) of Fig. 10, in which the symbols represent:- r - facet reflectivity; g - model gain; L - cavity section lengths; These expressions lead to the expressions (IV) and (V) of Fig. 10 for the output power for the m th mode for each facet. In these expressions, hν and d represent photon energy, and active layer thickness respectively. Having developed expressions for the output power, the wavelength of the output is obtained from the standard Fabry-Perot phase closure conditions, as in expression (VI) of Fig. 10. This expression incorporates the dependence of the refractive index on the carrier density. These expressions for power and wavelength of a given mode have been developed in terms of carrier concentrations N₁ and N₂. For this analysis to be of more benefit, it is necessary to carry out a transformation from the carrier density plane (N₁, N₂) to the current density plane (J₂, J₂,)or the current magnitude plane (I₁, I₂). This is achieved by using the equation of charge conservation given in expression (VII) of Fig. 10 which provides the required link between carrier density and current density. In expression (VII) R sp , Γ, and V g , represent total spontaneous emission rate, optical confinement factor and effective group velocity. A position independent average value for the photon density S m is used in this expression. S m is obtained by averaging the axial distribution of the photon density S m (z) over the cavity section length as in expression (VIII) of Fig. 10. Equations (VII) and (VIII) are then used to develop expressions for the two drive current densities J₁ and J₂. The above expressions are non linear and accordingly, an algorithm has been developed to obtain a numerical solution in a two-step procedure involving developing loci of constant power and wavelength in the N₁, N₂ carrier density plane and carrying out a plane transformation on the N₁, N₂ plane to the J₁, J₂ current density plane. Results are illustrated in Fig. 5 in which constant power contours are identified by the numeral 20 and constant wavelength contours are identified by the numeral 21. It will be appreciated that by representing these contours on the graph, the operation of the semiconductor laser transmitter 1 may be clearly seen. Although, needless to say, the contours illustrated vary according to temperature, facet reflectivities, chip material, structure and drive contact lengths. For any input drive condition, emitted light power and wavelength excursions may be easily determined. The results represented graphically in Fig. 5 are for the main lasing mode, however, by carrying out the same procedure for the other modes various features of device operation such as side-mode suppression can be investigated. Corresponding contours may alternatively be generated using a travelling wave analysis, insted of the Fabry-Perot analysis described above. It has long been appreciated that there are many advantages to transmitting signals in light form instead of electrically. Consequently, much work has been carried out in attempting to improve control of light signals so that they may contain more information and in a more readable form. Our invention resides in using our understanding of the operation of a semiconductor laser to provide a major improvement in control of light signals generated. The following is a description of some examples of circuits which are used to control light signals using the understanding of the operating characteristics of the semiconductor laser. These examples are not, however, exhaustive and many other applications are envisaged. Referring now to Fig. 6, there is illustrated a semiconductor laser assembly 30 including the laser transmitter 1 connected to a signal input circuit. The signal input circuit comprises DC bias current sources 31 and 32 for the drive contacts 5a and 5b respectively having isolation inductors 43 and 44. The signal input circuit also includes a varying, in this case the alternating current circuit 35 comprising a common signal input line 36 which is connected to an inverter 37 in series connection with a drive amplifier 39 and an isolating capacitor 41 and thence to the drive contact 5a. The common input signal line 36 is also connected to the drive contact 5b via a delay circuit 38 in series connection with a drive amplifier 40 and an isolation capacitor 42. In use, the input signal circuit supplies input drive current signals to each of the drive contacts 5a and 5b. Each drive current signal comprises a constant DC bias component and an alternating current component. The value of bias current is selected according to the region in the current density or current gain plane in which the laser transmitter 1 is to be operated. The alternating current components vary the magnitude of the drive current signals for control of wavelength and power of emitted light according to any desired format so that information may be encoded in the emitted light. In this embodiment, the alternating current components vary in mutual inverse relationship because they are derived from a common input signal which is inverted before transmission to one drive contact and is delayed by an amount equal to the time delay of the inverter 37 before transmission to the other drive contact. Variations of wavelength and power of emitted light may thus be represented by the arrowed line 45 of Fig. 7. It will be seen that because the alternating current components vary inversely the gain of one section increases as the gain of the other section decreases and vice-versa. The drive format is linear because the drive amplifiers 39 and 40 are linear in operation. Needless to say, the drive amplifiers may be modified to operate in a non-linear manner to allow the emitted light parameters follow the contours more closely. It will be seen in this embodiment that the light output is of substantially constant wavelength and varies between different intensity levels. Accordingly, if electrical data transmitted to the common input signal line 36 is in non-return to zero and invert (NRZI) form, the encoded information in the emitted light will be in return to zero (RZ) format. The semiconductor laser 1 may thus be used as an opto-electric digital switching unit for transcoding between different code formats. It is also envisaged, for example, that the laser transmitter 1 may, in this manner, be used for conversion between pulse duration modulation and pulse position modulation formats. In this case, the data address may be imposed on the laser output in the form of a time delay. If the data input on the common input signal line 36 has a bit interval of T, the output laser beam will be two short pulses of light separated by the interval T. If a receiver is only permitted to accept data after checking that this interval is correct for that particular destination, then this address forms part of the laser beam signal. It has been found, for example, that self-routing of 100M bit/second data to ten addresses can be achieved in this way. It will also be appreciated that the laser assembly 30 may be used as an active tunable filter for optical communication receivers. By appropriate control of the drive current signals I₁ and I₂ and also by injecting received light from another laser beam through one facet, the laser transmitter acts selectively on the wavelength spectrum and can amplify wanted and/or reject unwanted wavelengths. In this embodiment, control of the input signals has been achieved by control of the phase difference of the alternating current components. It is envisaged, however, that the bias current may be controlled or alternatively, the amplitude of the alternating current component may be controlled. Referring now to Fig. 8 there is illustrated an alternative construction of semiconductor laser assembly according to the invention indicated generally by the reference numeral 50. Parts similar to those described with reference to Fig. 6 are identified by the same reference numerals. The laser assembly 50 is similar to the laser assembly 30 except that in this embodiment additional terminals 51 and 52 are provided at the outputs of the inverter 37 and the delay circuit 38, respectively. With this arrangement, an electrical clock signal at frequency 2f may be inputted on the common input signal line 36. Accordingly, the drive contacts 5a and 5b receive mutually inverted clock signals as part of the alternating current component. The drive format caused by these clock signals is illustrated by the arrowed line 55 of the graph of Fig. 9. This drive format is similar to that illustrated in Fig. 7. In addition to these clock signals, however, the alternating current component also includes data signals D₁ and D₂ which are input at the terminals 51 and 52. These data input signals D₁ and D₂ are synchronised with the clock signal input on the line 36. A data bit D₁, if present, drives the laser beam output across power and wavelength contours as indicated by the arrow 56 of Fig. 9. Further, a data bit D₂, if present, will cause an excursion as indicated by the arrow 57 of Fig. 9, but only during the next half cycle of the clock signal. This process is repeated for each following clock period, thereby generating interleaved optical data which is in time division multiplexed form. Excursions of the data streams D₁ and D₂ may be set so as to provide the required optical peak-to-peak power, or to provide appropriate wavelength switching for data encoding or destination addressing. Furthermore, a mixture of both power and wavelength switching in the appropriate device could facilitate combined wavelength and time division multiplexing to achieve 4:1 MUX. This may be achieved alternatively by adding further drive contacts to give more than two cavity sections in the laser transmitter. In Fig. 8 there are also illustrated alternative terminals 53 and 54 for input of the data streams D₁ and D₂. The embodiments of Fig. 6 and 8 are mere examples of the manner in which the drive current signals may be controlled to take advantage of the manner in which wavelength and intensity of emitted light may be controlled. The drive format may be arranged in any desired manner to either provide for constant wavelength output or constant power output or variations in both power and wavelength. Indeed, because the relationship between output power and intensity and input current has been clearly defined, it is envisaged that the design of laser transmitter may be modified to vary these contours to suit any particular application. It is also envisaged that the input signal circuit may include a microprocessor which stores these contours or functions for the particular laser transmitter and controls the drive current signals according to these contours and the type of light desired.

Claims: A method of operating a semiconductor laser transmitter (1) of the type comprising a stimulated emission cavity (4) and at least two separate contacts (5a, 5b), each of which contact is associated with a region (10, 11) of the stimulated emission cavity (4), the method comprising the steps of injecting electrical signals to the contacts to generate optic signals characterised in that, the electrical signals are electronic data signals (I₁, I₂) having a magnitude sufficient to drive each associated cavity region (10, 11) above a stimulated emission threshold level to generate cross-coupled stimulated emission by interaction of photons (γ₁, γ₂) of each region with carriers (N₁, N₂) of the or each other region; and the electronic data signals (I₁, I₂) are varied to control optic intensity (P) and wavelength ( λ ) according to pre-defined characteristics representing response of optic intensity and wavelength to changes in cavity region carrier concentrations so that the laser transmitter (1) is operated as an opto-electronic data processor for processing the electronic data signals to generate optic data signals, the pre-defined characteristics being representable by constant wavelength and constant intensity contours as a function of carrier concentrations in the cavity regions. A method as claimed in claim 1, wherein the laser transmitter (1) is operated as a time division multiplexer (50) for data channels (D1, D2) delivered to each contact (5a, 5b) and synchronised with clock signals (CLK) also delivered to each contact at a rate approximately double that of each data channel, said clock signals driving the transmitter (1) between two carrier concentration states, at each of which state a substantial optic intensity excursion will occur for electronic data input at one contact only. A method as claimed in claim 1, wherein the semiconductor laser transmitter (1) is operated as a combined time division and wavelength multiplexer (50) for said data channels (D1, D2) and an additional data channel delivered to a contact by varying the clock signals delivered to each contact so that the optic data signals have a different wavelength at each operating state. A method as claimed in claim 3, wherein two data channels are delivered to each contact for 4 to 1 multiplexing. A method as claimed in claim 1, wherein the laser transmitter (1) is operated as a pulse duration to pulse position transcoder (30) by varying the electronic data signals to two of the contacts in a mutually inverse relationship to generate an optic intensity excursion of a complete cycle to form a pulse for each electronic data level change. A method as claimed in claim 5, wherein the electronic data signals are derived from a single data signal delivered directly to one contact and inverted before delivery to the other contact. A semiconductor laser transmitter assembly comprising:- a semiconductor laser transmitter (1) comprising a stimulated emission cavity (4) and having at least two separate contacts (5a, 5b), each of which is associated with a region (10, 11) of the stimulated emission cavity (4); characterised by means (31, 32) for injecting electronic data signals to the contacts to drive each associated cavity region (10, 11) above a stimulated emission threshold level thereof to generate cross-Coupled stimulated emission by interaction of photons (γ₁, γ₂) of each region with carriers (N₁, N₂) of the or each other region; and means (35, 51, 52) for varying the electronic data signals to control optic intensity and frequency according to pre-defined characteristics representing response of optic intensity and frequency to changes in cavity region carrier concentrations (N₁, N₂) so that the laser transmitter (1) is operated as an opto-electronic data processor for processing the electronic data signals to generate optic data signals, the pre-defined characteristics being representable by constant wavelength and constant intensity contours in response to the carrier concentrations in the cavity regions. A semiconductor laser transmitter assembly as claimed in claim 7, wherein the varying means comprises:- means (36-42) for delivering a clock signal (CLK) to each contact (5a, 5b) to drive the transmitter (1) between two carrier concentration states (N₁, N₂), at each of which state a substantial optic intensity excursion will occur for electronic data input at one contact only; and means (51, 52) for delivering a data signal (D1, D2) to each contact, the data signals being synchronised with the clock signals (CLK) so that the data processor acts as a time division multiplexer (50). A semiconductor laser transmitter assembly as claimed in claim 7, wherein the varying means comprises means (35) for varying the electronic data signals in a mutually inverse relationship, and means (31, 32) for delivering the signals to the contacts to generate an intensity excursion of a complete cycle for each electronic data level change, so that the assembly acts as a pulse duration to pulse position transcoder (30). An assembly as claimed in claim 9, wherein the varying means comprises a common terminal (36) for reception of the signal, means (38, 40, 42) for delivering the signal directly to one contact, and means (37, 39, 41) for inverting the signal before delivery to the other contact.

Inventors: O'DOWD RONAN FEARGUS, O'DOWD, RONAN FEARGUS
Applicants: ELECTRO OPTIC LOGIC LTD, ELECTRO-OPTIC LOGIC LIMITED
Application year: 1989
Main classification: H01S3
IPC codes: H04B10, H01S5, H04J14