This invention pertains to the field of digital optical communications.
A typical digital optical communications system includes a transmitter, an optical channel (e.g., optical fiber), and a receiver. Optionally, the optical channel may have one or more elements such as amplifiers to compensate for attenuation due to long distances and other dispersive effects. The transmitter is often constructed of a light source generating an optical carrier and an electro-optical modulator. A common electro-optical modulator is the Mach Zehnder (M-Z) modulator. The M-Z modulator operates by dividing incoming light into two optical paths. Into one or both paths is placed a material that, in response to a control voltage, alters the phase of the light traveling through it. The two paths are then optically recombined to generate the final output. Phase differences between the two paths cause destructive interference, operating to reduce the intensity of the output. When the two paths are exactly one-half wavelength out of phase (a radians), destructive interference is complete and no light exits the modulator. When the two paths are in phase constructive interference occurs and the light exiting is equal in intensity to the incident light (minus small system losses). FIG. 1 plots the percentage of transmitted light at different control voltages. The transfer characteristic of the M-Z modulator is a raised cosine. The difference in drive voltage between the minimum and maximum transmission points is called Vxcfx80 since it results in an optical phase change of xcfx80 radians. Vxcfx80 is generally stable, however, the absolute voltage of the optical minimum and maximum drift with time, temperature, and other phenomenon.
Typically, commercial M-Z modulators are driven with two electrical signals. The first is a data drive signal that modulates the phase difference between the two paths. The second is a bias voltage that controls the average phase difference between the paths. These electrical drive components can be combined externally to the M-Z, and applied to a single electrode set, or can be effectively combined within the M-Z modulator (on separate data and bias electrode sets). This separation of data and bias signals generally simplifies the overall electrical drive system. The bias voltage circuitry corrects for the relatively slow drift due to time, temperature, etc., freeing the high-performance data drive circuitry from addressing these effects. In some M-Z modulators, the data drive signal consists of a plurality of wires that may be driven differentially, redundantly or complementarily. The methods described in this patent apply equally to M-Z modulators with one or more data drive wires.
Some systems modulate multiple bands of electrical information onto a single optical carrier frequency. These systems are most efficient when the transmitter has a linear transfer characteristic. This is best achieved with the M-Z modulator by setting the bias point at a 50% transmission level (labeled xe2x80x98Axe2x80x99 on FIG. 1) and using the data drive signal to vary the transmission symmetrically about this point. Additional linearity can be achieved by limiting the data drive signal variance to less than Vxcfx80.
Other systems modulate the optical carrier with a single channel of digital electrical information. Increased data transmission rates are accomplished by increasing the electrical clock rate as well as the use of multiple modulators operating on multiple optical carriers of different frequencies (i.e., Wavelength Division Multiplexing). In these systems, linearity of the transmitter transfer characteristic is unimportant. Rather, the ability of the receiver to distinguish optical xe2x80x981xe2x80x99 from optical xe2x80x980xe2x80x99 must be maximized. We will call the ratio of the amplitude of the optical xe2x80x981xe2x80x99 to the amplitude of the optical xe2x80x980xe2x80x99 the extinction ratio. The higher the extinction ratio of the transmitter, the lower the required sensitivity of the receiver. The maximum extinction ratio is achieved by providing a data drive signal variance of Vxcfx80 combined with appropriate biasing. The term xe2x80x981xe2x80x99 level or xe2x80x980xe2x80x99 level is used to indicate the optimal optical amplitude at the transmitter when sending a xe2x80x981xe2x80x99 bit or a xe2x80x980xe2x80x99 bit to the receiver.
In prior-art systems, the data drive signal varies symmetrically and the bias voltage is maintained at the 50% level. Many bias voltage regulation schemes are known, one example is U.S. Pat. No. 6,046,838 titled, AUTOMATIC BIAS CONTROL FOR ELECTRO-OPTIC MODULATORS. In general, two prior-art bias methods are known. Both techniques rely on injecting a low frequency control tone into the M-Z modulator, extracting a resulting component of the control tone from the output of the M-Z modulator and adjusting the bias voltage by processing the extracted component and optionally, the original control tone.
In one technique, a control tone is modulated onto the bias voltage. The bias voltage is adjusted to maximize the detected control tone component. FIG. 6 shows an example of this system. Laser 700 generates an optical carrier transmitted through M-Z modulator 710. M-Z modulator 710 modulates the carrier by combining data drive signal 760 and bias voltage 770. Bias voltage 770 is generated by combining the output of control tone oscillator 750 and feedback control 730. The output of the M-Z modulator 720 is monitored by photo-detector 720. Feedback control 730 extracts from photo-detector 720 the control tone component that is combined with the output of control tone oscillator 750 to generate the necessary bias adjustment.
In another technique, the data drive signal is modulated by a control tone. The extracted control tone is multiplied by the original control tone to generate an error signal. Feedback correction of the bias voltage is done using well-known control techniques like proportional and integrative.
Increasing the transmitted data rate faces two difficulties. First, the shorter electrical wavelength of the modulating signal shortens the practical active length of the M-Z waveguide causing Vxcfx80 to increase. Second, it becomes increasingly difficult to achieve a particular voltage swing at higher electrical clock rates. These two difficulties combine to lower the actual data drive voltage swing below Vxcfx80, reducing the extinction ratio. When the data drive voltage swing is less than Vxcfx80, the system is said to be underdriven. One response to this effect is to increase the sensitivity required of the receiver. However, there are limits to receiver sensitivity due to a number of factors, including electrical noise in the photo-diode, optical noise injected by optical amplifiers in the transmission path and others. Consequently, it is desirable to maximize the extinction ratio of an M-Z modulator when the data drive voltage swing is less than Vxcfx80. The optimal extinction ratio for an underdriven M-Z modulator is achieved when the bias point is not at the 50% transmission level of the prior-art.
A goal of the invention is maximize the extinction ratio when the M-Z modulator is underdriven.
The amplitude of control tone modulation of the zero and one levels of the M-Z control voltage is independently controlled. Detecting the results of the asymmetric control tone modulation allows the system to better control the extinction ratio.
In a preferred embodiment, all of the control tone modulation is placed onto the zero level. The system adjusts the M-Z bias point to maximize the extinction ratio.