Abstract:
An electro absorption modulator in which the photocurrent generated by the absorption of light is monitored and the bias voltage applied to the electro absorption modulator is varied in order to vary the photocurrent and thence the extinction ratio of the electro absorption modulator.

Description:
FIELD OF THE INVENTION 
     This invention relates to the field of semiconductor optical modulators and in particular to electro absorption modulators that are commonly used to provide external modulation to optical signals. 
     DISCUSSION OF THE BACKGROUND ART 
     Optical transmission systems have seen dramatic increases in data transmission rates, with 10 Gb/s systems in use in many SDH networks, with 40 Gb/s systems under development. One technique that has been used to obtain such data transmission rates is external modulation of optical sources. Conventionally, optical sources such as laser diodes have been directly modulated by supplying the modulating signal to an electrode connected to the active region of the laser such that the output of the laser varies with the modulating signal. The main drawback with this technique is that the data transmission rates are limited by the photonic transitions that govern the population inversion and radiative decay. In comparison, external modulation relies upon an optical device that can be switched between an attenuating state and a substantially non-attenuating state such that data can be modulated onto the constant output of an optical source. One device that is commonly used to provide external modulation is an electro absorption modulator (EAM), the structure and operation of an example of an EAM is described in EP-B-0 143 000. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the invention there is provided an optical modulator comprising an input to receive an electrical input signal and an output to present an electrical output signal characterised in that the modulator additionally comprises control means to vary the electrical input signal in response to the electrical output signal. Preferably the optical modulator is an electro-absorption modulator. The electrical input signal may be a dc bias voltage. The electrical output signal may be an ac signal and preferably varies with the absorption within the electro-absorption modulator. 
     According to a second aspect of the invention there is provided an optical signal generator comprising an optical source and an optical modulator as described above. Preferably the optical source is a distributed feedback laser. 
     According to a third aspect of the invention there is provided a method of operating an optical modulator, the method comprising the steps of: (i) applying an electrical control signal to the modulator; (ii) applying an electrical modulation signal to modulate a received optical signal; (iii) receiving an electrical output signal from the modulator, the method being characterised by the step of (iv) varying the electrical control signal in response to the electrical output signal. Preferably the optical modulator is an electro-absorption modulator. The electrical control signal may be a dc bias voltage. The electrical output signal may be an ac current and preferably the electrical output signal varies with the absorption incurring within the modulator. 
     Preferably the electrical control signal is varied to increase the magnitude of the electrical output signal. The electrical control signal may be varied to maintain the magnitude of the electrical output signal at or above a predetermined threshold. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention will now be described, by way of example only, with reference to the following Figure in which: 
     FIG. 1 shows a schematic depiction of a semiconductor optical modulator apparatus according to the present invention; 
     FIG. 2 is a graphical depiction of the relationship between EAM photocurrent and laser injection current; and 
     FIG. 3 is a graphical depiction of the relationship between EAM photocurrent and the EAM bias voltage. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows a schematic depiction of a semiconductor optical modulator apparatus  10  that comprises optical source  12 , electro absorption modulator (EAM)  14 , control unit  30 , modulator unit  20  and bias unit  22 . EAM  14  comprises first and second terminals  16 ,  18  and control unit  30  comprises detector unit  34  and feedback unit  32 . The optical source  12  provides an optical signal having a substantially constant optical power output that is received within the EAM  14 . Typically the optical source is a laser and distributed feedback (DFB) lasers have been found to be particularly suitable in this application. 
     The EAM typically comprises a reverse biased pin junction that has an absorption bandedge. The wavelength at which this absorption bandedge is effective can be controlled by a modulating current such that, under the correct conditions, a ‘0’ (or low) modulating signal causes the optical signal from the optical source to be partially absorbed by the EAM and a ‘1’ (or high) modulating signal causes the EAM to allow the signal to pass through substantially unabsorbed. The modulated optical signal can then be launched into an optical fibre for onward transmission. 
     In the modulator apparatus  10  a modulating ac signal is fed from modulator unit  20  to first EAM terminal  16 . Furthermore a dc biasing voltage is fed from bias unit  22  to first EAM terminal  16  to control the operation of the EAM. One EAM variable that is dependent upon the bias voltage is the extinction ratio, which is the ratio between a partially absorbed signal (when the EAM is in a blocking state) and a substantially unabsorbed signal (when the EAM is in a non-blocking state). Extinction ratios are usually within the range of 3 to 20 dB with a typical value being 10 dB. 
     When an EAM is modulated using an ac signal, a small ac photocurrent is generated in the electro-absorption region in proportion to the amount of light absorbed within the EAM. The insight of the present inventor is that this photocurrent can be varied to improve the performance of the EAM. 
     The photocurrent leaves the EAM at second EAM terminal  18  and is fed to detector unit  34  within control unit  30 . Detector unit  34  measures the magnitude of the ac photocurrent and passes the data to feedback unit  32 . The inventor determined that the maximum extinction ratio occurs when the photocurrent is at its highest and thus the feedback unit  32  sends control signals to the bias unit  22  to adjust the bias voltage in order to increase the ac photocurrent. The bias voltage can be controlled so as to maximise the ac photocurrent (and hence to have a substantially maximal extinction ratio), or alternatively the bias voltage can be controlled to ensure that the extinction ratio remains above an upper threshold. 
     FIG. 2 shows the variation of EAM photocurrent with laser injection current. Once the threshold level of the laser has been surpassed the photocurrent generated within the EAM increases proportionally with the laser current. FIG. 3 shows the variation in EAM photocurrent, for a constant input light level from a laser, with bias voltage. FIG. 3 shows that between approximately −0.2 V and −1 V (the EAM is negatively biased, hence the polarity of the voltages) there is a plateau, with the photocurrent generally increasing as the bias voltage increases from −1 V to −2 V, with a peak at around −1.5 V. The optimal bias voltage is one at (or near to) this peak in the bias voltage characteristic, such that the photocurrent will have its maximum value (or a value substantially identical to its maximum value). In the alternative, for the example shown in FIG. 3, the bias voltage could be controlled so as to maintain the photocurrent at or above 1.3 mA, for example. Clearly, the above examples relate to one particular device being operated under a certain set of parameters and different modulators, and different designs of modulators, will have different characteristics to those shown in FIGS. 2 &amp; 3. 
     It will be readily understood that the arrangement described above with reference to FIG. 1 could be an integrated opto-electronic device, i.e. the laser and the EAM could be fabricated together, or two discrete components could be co-located. In the alternative, the laser and the EAM could have different locations within a piece of equipment with the output from the laser being delivered to the EAM by an optical waveguiding structure, such as an optical fibre. The present invention is suitable for use with any type of EAM, regardless of the materials used to construct the EAM or the wavelength(s) at which it operates.