Abstract:
An optical emitter includes an electro-optical modulator which comprises N modulator units optically coupled in series and having N separate control electrodes. The modulator is controlled by an electronic control circuit which delivers N identical electrical signals at N separate outputs, each of which is connected to a respective control electrode of one of the N modulator units.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is based on French Patent Application No. 01 05 758 filed Apr. 27, 2001, the disclosure of which is hereby incorporated by reference thereto in its entirety, and the priority of which is hereby claimed under 35 U.S.C. §119. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the invention  
           [0003]    The present invention relates to high bit rate optical transmission, in which it is important to maximize optical component bandwidth. To this end, in the case of an optical emitter, for example a laser, one solution is to reduce the intrinsic capacitance of the optical modulator that is generally associated with said emitter. The invention is more specifically concerned with monolithic integrated optical components, i.e. components fabricated on a single integrated circuit chip.  
           [0004]    2. Description of the Prior Art  
           [0005]    There are various types of optical modulator. Electro-absorption modulators amplitude-modulate the optical wave by displacing the absorption peak of the material constituting the waveguide by means of a control voltage. Mach-Zender modulators, comprising two branches, modulate the phase of the optical wave in at least one branch to produce interference at the output of the two branches. A control voltage is applied to at least one branch, the other of which can be grounded.  
           [0006]    An optical electro-absorption modulator (EAM) is shown diagrammatically in FIG. 1. This kind of modulator is well known in the art. It comprises a waveguide  11  taking the form of a ridge (mesa) on a semiconductor material (for example InP) substrate  10 . The waveguide  11  includes an intrinsic semiconductor material active layer  12  between two semiconductor material layers each doped with a different type of carrier. A control voltage is applied to the guide  11  to displace the absorption peak of the waveguide  11  toward the working wavelength and amplitude-modulates the wave passing through the guide.  
           [0007]    The intrinsic capacitance C of an EAM is defined by the following equation:  
             C=εε   0 ( I×L )/ e    
           [0008]    in which I and L are respectively the width and the length of the waveguide  11  of the modulator, e is the thickness of the active layer  12  of the waveguide  11 , and ε and ε 0  are dielectric constants.  
           [0009]    To reduce the capacitance C, it is necessary either to reduce I or L or to increase e. Physical and optical constraints apply to these parameters.  
           [0010]    First of all, the width I of the wavelength  11  is fixed by the optical constraints on propagation of waves in the guide  11 . Next, the thickness e of the active layer  12  is fixed by fabrication constraints, to be more specific epitaxy constraints. Moreover, by increasing the thickness e, the efficiency of modulation is reduced, which is not desirable. Finally, the length L of the waveguide  11  can be reduced to reduce the capacitance C. However, reducing the length L reduces the absorption, thereby degrading the performance of the component.  
           [0011]    In the case of a discrete Mach-Zender modulator, the problem of reducing the capacitance is the same for each branch, and the solution proposed by the present invention is applied in the same manner.  
           [0012]    The object of the present invention is to alleviate the drawbacks cited, i.e. to reduce the capacitance of a modulator without degrading its optical performance.  
           [0013]    To this end, the invention proposes to produce an optical emitter including an electro-optical modulator comprising a plurality of modulator units each controlled by the same electrical control signal, each unit having a reduced waveguide length, and therefore a commensurately reduced capacitance.  
         SUMMARY OF THE INVENTION  
         [0014]    To be more specific, the invention provides an optical emitter including an electro-optical modulator which comprises N modulator units optically coupled in series and having N separate control electrodes and is controlled by an electronic control circuit which is adapted to deliver N identical electrical signals at N separate outputs each of which is connected to a respective control electrode of one of the N modulator units. Depending on the application, the modulator units are either electro-absorption modulators or discrete Mach-Zender modulators.  
           [0015]    In respective different embodiments the electronic control circuit is a multiplexer with M inputs and N synchronized outputs and an amplifier with one input or M inputs and N synchronized outputs.  
           [0016]    According to an advantageous feature of the invention the optical emitter takes the form of a monolithic integrated component.  
           [0017]    According to another feature of the invention the electronic control circuit is integrated into the electro-optical modulator.  
           [0018]    Other features and advantages of the invention will become clearly apparent after reading the following description, which is given by way of illustrative and non-limiting example and with reference to the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    [0019]FIG. 1, already described, is a diagram of a prior art electro-absorption modulator.  
         [0020]    [0020]FIG. 2 shows diagrammatically the structure of an optical emitter according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0021]    The objective of the present invention is to provide an optical emitter including an electro-optical modulator having the required high bit rate data transmission performance.  
         [0022]    As already indicated, one solution to reducing the capacitance of a modulator is to reduce the length of its waveguide.  
         [0023]    The present invention therefore proposes to provide a modulator comprising a plurality of modulator units M N  of reduced length, and therefore of reduced capacitance, and to place them optically in series to alleviate the disadvantage of the reduced absorption of the optical wave propagating in the modulator. The global absorption of the optical wave emitted by the emitter is then the sum of the absorptions in each of the modulator units placed in series, and the bandwidth is widened because of the reduced intrinsic capacitance of each unit.  
         [0024]    [0024]FIG. 2 is a diagram showing the implementation of the invention. In this example there are two modulator units M 1  and M 2 , but the invention applies in the same manner to any number N of modulator units disposed in series.  
         [0025]    An emitter  100 , for example a laser, must emit a modulated optical signal S m  from the output, at wavelength λ c , of a continuous source. The emitter  100  includes an electro-optical modulator  30  comprising N (N=2 in this example) modulator units M N  (M 1 , M 2  in this example) disposed optically in series.  
         [0026]    In a preferred embodiment, the modulator units M N  are electro-absorption modulators (EAM). However, the invention applies equally to modulator units MN comprising discrete Mach-Zender (MZ) modulators.  
         [0027]    An electrical control signal Q is applied to the waveguide of each modulator unit M N . The electrical control signals Q for each modulator unit M N  must be identical, with a time-delay matched to the time-delay of the light signal between the units M N . The modulator units M N  of the modulator  30  are thus electrically disposed in parallel.  
         [0028]    To this end, the modulator  30  is controlled by an electronic circuit  50  with an input E and N synchronized outputs Q. This kind of control circuit  50  is known in the art and can readily be produced by the person skilled in the art. For example, it can be a multiplexer with one input and N outputs, or an amplifier circuit receiving an electrical input signal with a low amplitude (0.4 Volts, for example) and producing at its output N synchronized electrical control signals Q with an amplitude from 3 to 4 Volts, for example. An example of this kind of amplifier circuit is described in the publication “Short optical pulse generation at 20 GHz repetition rate using integrated laser—modulators—amplifier” by G. Martin, E. Vergnol, A. Carenco, and A. Ramdane, ECOC 2000, Munich, September 2000.  
         [0029]    According to the invention, the modulator  30  and the electronic control circuit  50  are integrated on the same integrated circuit chip. In particular, the modulator can be integrated into the emitter  100 . This monolithic integration of the optical component  100  avoids the problems of optical wave phase shift. This monolithic integration of the modulator units M N  with the emitter is possible because each modulator unit is controlled by the same parallel electrical signal, enabling the use of a common ground electrode for the modulator units and the emitter.  
         [0030]    Thus, in the embodiment of the invention shown, a modulator of length L and capacitance C is replaced by two modulator units M 1  and M 2  of length L/2. As a result the overall length of the modulator  30  is 2×L/2, that is to say L, and the absorption is therefore not penalized, but the intrinsic capacitance of the modulator M is reduced to C/2 by the series connection of the two modulator units M 1  and M 2 , and the bandwidth of the emitter  100  is therefore enlarged.