Abstract:
Methods and apparatus are described for modulating an optical signal using electroabsorption in conjunction with an optical interferometer. Phase-shift keying modulation can be achieved with lower amplitude modulator drive signals than conventional methods by splitting the signal to be modulated into multiple optical modes and interferometrically combining the modes after modulating at least one of the modes with an EAM. Using the present invention, the extinction ratio performance of ASK can be significantly improved for a given drive voltage or a desired extinction ratio can be achieved with a substantially lower drive voltage. Hence, the elecro-optic bandwidth of EAMs can be enhanced by overcoming the trade-off relationship between extinction ratio and bandwidth. Furthermore, the present invention can be used to generate other modulation formats, such as QPSK or QAM, with much lower drive voltages, thereby reducing the cost and power consumption of the high-speed drive electronics for the modulation.

Description:
FIELD OF THE INVENTION  
       [0001]    The present invention relates to the field of high-speed optical communications, and more specifically to electroabsorption modulation methods and apparatus. 
       BACKGROUND INFORMATION  
       [0002]    Semiconductor electroabsorption modulators (EAMs) offer the advantages of low drive voltage, small form factor, and integratability with active optical elements. EAM-integrated lasers are widely used in metro-distance communication networks. EAMs are also key building blocks for highly integrated photonic circuits. The low drive voltage requirement of EAMs is especially promising for emerging high bit-rate (&gt;100 Gb/s) communications, where the wide-band electrical amplification required for driving lithium niobate modulators (LNMs) is difficult to achieve. 
         [0003]    In contrast to LNMs, which typically are operated using phase modulation combined with interferometric structures, EAMs have been used as single amplitude modulation elements. Considering the importance of phase shift keying (PSK) modulation, including quadrature phase shift keying (QPSK), and quadrature amplitude modulation (QAM) in high bit-rate, high spectral-efficiency transmission, it would be desirable to use EAMs for a wider range of modulation formats other than amplitude modulation. 
       SUMMARY OF THE INVENTION  
       [0004]    The present invention is directed to modulating optical signals using electroabsorption in conjunction with an optical interferometer. In an exemplary embodiment, phase shift keying (PSK) modulation is performed using a polarization-sensitive electroabsorption modulator (EAM) operating interferometrically. The PSK modulation is achieved by interferometrically combining the transverse electric (TE) and transverse magnetic (TM) modes of an optical signal propagating through the EAM. 
         [0005]    In a further exemplary embodiment, interferometric operation of an EAM is used to perform amplitude shift keying (ASK) modulation, such as on-off keying (OOK). For ASK modulation, interferometric operation of an EAM provides significantly improved extinction ratio performance for the same drive voltage, or alternatively it allows EAMs to be operated at much lower drive voltages for a desired extinction ratio. The present invention can be used to enhance the electro-optic bandwidth of EAMs, overcoming the trade-off relationship between extinction ratio and bandwidth. 
         [0006]    In further exemplary embodiments, EAMs incorporated into an interferometer structure, such as a Mach-Zehnder interferometer (MZI). In one such embodiment, an EAM is included in each arm of an MZI with the outputs of the EAMs being interferometrically combined. Such EAM-MZI structures can be combined to provide various modulation formats, in accordance with the present invention. 
         [0007]    In addition to the aforementioned, the present invention can be used for a wide variety of modulation formats, including, for example, QPSK and QAM. 
         [0008]    The aforementioned and other features and aspects of the present invention are described in greater detail below. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0009]      FIG. 1  is a block diagram of an exemplary embodiment of an interferometric modulation device comprising an electroabsorption modulator (EAM) in accordance with the present invention. 
           [0010]      FIGS. 2A-2C  graphically illustrate interferometric operation of an EAM in accordance with the present invention. 
           [0011]      FIG. 3  shows an exemplary embodiment of an EAM-Mach-Zehnder interferometer (EAM-MZI) structure in accordance with the present invention. 
           [0012]      FIGS. 4A and 4B  illustrate OOK and PSK signals generated with the EAM-MZI structure of  FIG. 3 . 
           [0013]      FIG. 5  shows an exemplary embodiment of an EAM-MZI structure incorporating a semiconductor optical amplifier (SOA), in accordance with the present invention. 
           [0014]      FIG. 6  shows an exemplary embodiment of a modulator incorporating multiple EAM-MZI structures, in accordance with the present invention. 
           [0015]      FIGS. 7A and 7B  show eye diagrams of a PSK signal generated by an exemplary embodiment of a modulation device in accordance with the present invention, before and after demodulation, respectively. 
           [0016]      FIGS. 8A and 8B  show eye diagrams of on-off keying (OOK) signals generated by an EAM without and with interferometric operation, respectively. 
           [0017]      FIG. 9  shows graphs of the extinction ratio and receiver sensitivity for OOK signals generated by an EAM with and without interferometric operation. 
       
    
    
     DETAILED DESCRIPTION  
       [0018]      FIG. 1  is a block diagram of an exemplary embodiment of a system  100  comprising an electroabsorption modulator (EAM) operating interferometrically in accordance with the present invention. In the system  100 , a laser light having an electric field designated E L  is launched via a polarization controller  110  into an EAM  120 . The launched laser light may be any suitable laser light and may be modulated or unmodulated, including, for example, a continuous wave (CW) laser or an optical pulse train. The polarization controller  110  controls the launch angle θ of the laser. The launch angle θ is the angle of the electric field E L  of the laser with respect to the transverse electric (TE) axis of the EAM  120 . An analyzing polarizer  130  is arranged at the output of the EAM  120  with an orientation angle of φ with respect to the TE axis of the EAM  120 . 
         [0019]      FIG. 2A  graphically illustrates the principle of operation of the above-described arrangement. The electric field E L  is shown as a vector which is decomposed along the TE and transverse magnetic (TM) axes of the EAM  120 . The TE and TM components of the electric field E L  are shown in  FIG. 2A  as the vectors E TE (t) and E TM (t), respectively. The TE and TM components E TE (t) and E TM (t) are projected onto the analyzing polarizer  130 , which interterferometrically combines the respective projections. 
         [0020]    The EAM  120  amplitude modulates the laser in accordance with a data stream (DATA). As the laser is modulated with data and its electric field E L  varies with time, the TE and TM component fields E TE (t) and E TM (t) and their projections onto the analyzing polarizer  130  will vary accordingly, as shown along the t-axis in  FIG. 2A  by the traces  210  and  220 . 
         [0021]    As can be readily appreciated from  FIG. 2A , the relative magnitudes and modulation depths of the two projected fields  210 ,  220  can be adjusted by controlling the launch and analyzer orientation angles, θ and φ, respectively. 
         [0022]    EAMs based on quantum wells show preferential TE-polarization absorption unless the anisotropy is relieved by tensile strains. As shown in  FIG. 2A , the TE modulation depth is substantially larger than the TM modulation depth. For example, a commercial EAM device such as the OKI OM5753C-30B has a TE modulation depth approximately 9 dB larger than the TM modulation depth for a 2V drive signal amplitude and a −1V bias voltage. 
         [0023]    As shown in  FIGS. 2B and 2C , respectively, PSK modulation or amplitude shift keying (ASK) modulation, such as on-off keying (OOK), can be achieved by properly adjusting the angle parameters, θ and φ, which affect the TE and TM projections. Thus, while the “raw” modulation depths of the TE and TM modes are set by the properties of the EAM and the bias and drive voltages, the present invention makes it possible, by interferometric operation, to achieve PSK or improve the amplitude modulation depths, i.e. extinction ratio, by adjusting the angle parameters θ and φ. 
         [0024]    As shown in  FIG. 2B , PSK modulation can be achieved when the mean values of the TM and TE projection fields  210  and  220  are roughly equalized. 
         [0025]    As shown in  FIG. 2C , OOK modulation with an enhanced extinction ratio can be achieved when the low levels of the TE and TM projection fields  210  and  220  are equalized. 
         [0026]    In further exemplary embodiments, the operational principle of the present invention can be realized utilizing other arrangements, including, for example, a Mach-Zehnder interferometer (MZI) with an EAM in each arm, as shown in  FIG. 3 . In addition to reduced polarization sensitivity, such an embodiment may be beneficial in reducing excess loss, as discussed below. 
         [0027]    As shown in  FIG. 3 , a MZI  300  comprises a first EAM  301  in a first arm and a second EAM  302  in a second arm, with a phase shifter  305  arranged in the second arm, in line with the EAM  302 . (As can be appreciated, a phase shifter can be arranged in the first arm instead of the second arm, or in both arms, so long as the appropriate phase relationship between the two arms is provided.) The EAMs  301  and  302  are driven by respective drive signals, Drive Signal  1  and Drive Signal  2 . A CW laser is provided to the EAMs  301 ,  302  via an input coupler (or splitter)  303 . The outputs of the EAMs  301 ,  302  (via the phase shifter  305 ) are combined by an output coupler  304 . 
         [0028]    Various drive signal configurations may be employed depending on the type of modulation format, the specification of the EAMs, and the coupling ratio of the input and output couplers  303 ,  304 . 
         [0029]    In a first such configuration, one of the EAM drive signals (e.g., Drive Signal  1 ) is the data signal to be modulated. The other EAM drive signal (e.g., Drive Signal  2 ) is a DC bias voltage which is selected so that the CW transmission through the corresponding EAM ( 302 ) cancels the low level of the modulated signal in the other EAM ( 301 ), as shown in  FIG. 4A . In  FIG. 4A , the dotted trace represents an NRZ-OOK modulated signal in the first EAM  301 , the dashed trace represents the CW transmission signal attenuated by the second EAM  302  and the solid trace represents the output signal resulting from the destructive interference of the aforementioned signals. Such a configuration provides an improvement in extinction ratio over OOK generated by an individual EAM operating conventionally. 
         [0030]    The DC bias voltage to be applied (as Drive Signal  2 ) will depend on the dynamic extinction ratio of the first EAM  301  and the coupling ratios of the couplers  303 ,  304 . The coupling ratios are preferably selected to reduce the transmission loss of the EAM-MZI structure. Factors which affect the optimal coupling ratio include the raw extinction ratio of the first EAM  301 , the intended modulation format, and the insertion losses of the two EAMs. If the goal is to increase the extinction ratio of OOK, then the power splitting ratio of the input coupler  303  should be ε/(1+ε), where ε is the raw extinction ratio if the second EAM  302  has the same insertion loss as the first EAM  301  and no voltage is applied to the second EAM  302 . Other coupling ratios may be used if the second EAM  302  is to be biased to control the propagation loss. Because it may be difficult to know all of the device parameters in advance, a variable coupler may be advantageous. The coupling ratio can be fixed, however, and the extinction ratio optimized by adjusting the bias voltage to the second EAM  302 . 
         [0031]    This single data drive signal configuration can also be used to generate PSK signals by adjusting the DC bias voltage (Drive Signal  2 ) accordingly. 
         [0032]    In a second drive signal configuration, Drive Signal  1  is the data signal D and Drive Signal  2  is the complementary data signal  D . This configuration can be used, for example, to generate Binary PSK signals (BPSK) with reduced optical loss. The signals in this driving configuration are shown in  FIG. 4B  in which the dotted trace represents the OOK signal output of the first EAM (driven by the data signal D), the dashed trace represents the OOK signal output of the second EAM (driven by the complementary data signal  D ) and the solid trace represents the PSK signal resulting from the destructive interference of the two EAM output signals. This dual drive scheme reduces the optical loss by at least 6 dB in comparison to PSK generation with a single data drive signal. 
         [0033]    A comparison of  FIGS. 2A and 2B  with  FIGS. 4A and 4B  shows the benefit of the EAM-MZI structure in reducing the optical loss resulting from the destructive interference. For generating PSK, the arrangement represented by  FIG. 4B  is superior to that of  FIG. 2B  because at least 6 dB less optical power is subtracted during the destructive interference. Similarly,  FIG. 4A  is superior to  FIG. 2C  in generating extinction-enhanced OOK signals as less optical power is lost during the destructive interference. 
         [0034]    The destructive interference employed in generating signals in accordance with the principles of the present invention results in optical loss. This optical loss, however, can be compensated for by providing amplification, as in the exemplary device shown in  FIG. 5 . In the device of  FIG. 5 , a semiconductor optical amplifier (SOA)  510  is preferably integrated into an EAM-MZI device  500  to compensate for the aforementioned optical loss. In order to minimize the impact of any pattern dependence of the SOA, it is preferable to place the SOA at the output of the modulator (as shown) for PSK generation, or at the input of the modulator for OOK generation. The ability to readily integrate SOAs with the EAM-based modulators of the present invention is an advantage over other devices, such as lithium niobate modulators, for example. 
         [0035]    Although the foregoing description was made with reference to non-return-to-zero (NRZ) signal formats, the same optical modulators can also be used with RZ formats as well. An exemplary arrangement of obtaining the drive signal for RZ format generation consists of an electronic AND circuit, performing the Boolean operation between the NRZ data signal and a clock signal of the line rate. The product of the AND operation can be used to drive the EAM-MZI modulators to generate RZ-format signals. EAM-based modulators such as those of the present invention are better suited for employing such a technique than are lithium niobate modulators because of the low drive voltage as well as the optical transmission characteristic, which is highly nonlinear as a function of the drive voltage. At high bit rates, the Boolean AND operation tends to introduce signal distortion, especially for large amplitude signals. Lithium niobate modulators are more susceptible to such distortion due to their fairly linear transmission response with respect to the drive voltages. 
         [0036]    The generation of more complicated modulation formats can be achieved by combining multiple EAM-interferometer structures in accordance with the present invention. An exemplary embodiment of a modulator that can be used to generate QPSK signals is shown in  FIG. 6 . The modulator shown in  FIG. 6  comprises a first EAM-MZI structure  610  and a second EAM-MZI structure  620  arranged in an MZI structure  600 . The EAM-MZI structures  610  and  620  each comprises a π phase shifter  611 ,  612 , respectively, and each is driven by a data signal and its complement (A, Ā and B,  B , respectively) to generate corresponding BPSK modulated signals. The BPSK signals are combined to produce a QPSK signal by introducing a relative phase shift of π/2 between the two signals. Such a phase shift is provided by the phase shifter  625 . 
         [0037]    QPSK generation using EAM interferometers, such as by the exemplary embodiment of  FIG. 6 , enjoys substantially reduced drive voltage requirements, thereby reducing the cost and power consumption of the drive electronics. 
         [0038]    The device of  FIG. 6  can be used for generating other signal formats, including, for example, 8-QAM signals. To do so, one of the EAM-MZIs  610 ,  620  (but not both) is effectively disabled for a bit period, such as by applying the same voltage to both drive signal inputs of the EAM-MZI to be disabled. 
         [0039]    The performance of exemplary embodiments of EAM-based interferometric devices in accordance with the present invention will now be described. 
         [0040]    In an exemplary embodiment of a device such as that of  FIG. 1 , CW light with a wavelength of 1553 nm is used and its launch angle into the EAM ( 120 ) is controlled using an HP8169A polarization controller ( 110 ). The polarization of the output of the EAM is controlled using a paddle polarization controller and the output of the EAM is analyzed by a fiber-optic polarizer. The EAM has a 3-dB bandwidth of 30 GHz and the amplitude of the drive signal is 2.6 volts. 
         [0041]      FIG. 7A  shows the electrical eye diagram of a 40-Gb/s PSK signal generated by such a system.  FIG. 7B  shows the eye diagram of the PSK signal after demodulation by a 25-ps delay interferometer. 
         [0042]    The demodulated signal was then electronically de-multiplexed and bit error rates (BER) were measured at 10 Gb/s after an erbium-doped fiber amplifier (EDFA) with a noise figure of 5.6 dB and an optical filter with 1-nm bandwidth. The receiver sensitivity at a BER of 10 −9  is −24.3 dBm. This is approximately 3 dB worse than that obtained with PSK generated using a lithium niobate modulator biased at a null. As evidenced in the demodulated eye diagram ( FIG. 7B ), the penalty is mostly due to the waveform distortion caused by the incomplete interference between the TE and TM fields, which have non-identical temporal shapes owing to the different responses of the two modes to the drive signal. 
         [0043]    With respect to the generation of ASK modulated signals, the extinction ratio of an ASK signal conventionally modulated by an EAM is related to the amplitude of the drive signal. Operation with a lower-voltage drive is preferable for reducing the power consumption and complexity of the driving electronics. 
         [0044]      FIG. 8A  shows the electrical eye diagram of OOK signals obtained directly from an EAM operating conventionally and not in an interferometric mode. In this case, the maximum extinction ratio (ER) is achieved when the CW laser is aligned with the TE axis of the EAM, which in this case is 5.7 dB for a 1.1 V drive voltage. 
         [0045]      FIG. 7B  shows a clear improvement in ER when operating the EAM in an interferometric mode with the angles θ and φ optimized. (For the exemplary data shown in  FIG. 7B , the angles were 29 degrees.) The improved ER is measured to be 12.5 dB for the same drive voltage of 1.1 V. 
         [0046]      FIG. 9  shows the extinction ratio (ER) and receiver sensitivity (RS) as function of drive voltage, for a BER of 10 −9 , for OOK signals generated by an EAM operating non-interferometrically and interferometrically. The open circles and squares represent the ER and RS, respectively, for the non-interferometric EAM and the solid circles and squares represent the ER and RS, respectively, for the interferometric EAM. For non-interferometric operation, the best ER and RS are achieved when the input laser light is aligned with the TE axis of the EAM. With interferometric operation in accordance with the present invention,  FIG. 9  shows an ER improvement of 8.7 dB and an RS improvement of 7 dB for a drive voltage of 0.8V. Advantageously, 0.8V, 40-Gb/s drive signals can be generated in accordance with the present invention, without using an electrical amplifier. 
         [0047]    It is understood that the above-described embodiments are illustrative of only a few of the possible specific embodiments which can represent applications of the invention. Numerous and varied other arrangements can be made by those skilled in the art without departing from the spirit and scope of the invention.