Optical Fiber Sources
The most commonly used optical source is the LASER diode (Light Amplification by Stimulated Emission of Radiation). Physically, a laser converts electrical current to light, which is in turn coupled into an optical fiber. An important characteristic of a laser is its threshold current. A typical current Vs optical output power is shown in figure 6.
Laser Modulation
Fiber communication systems typically employ some form of digital communication technique that requires the laser to be switched ON to transmit a digital 1 and switched OFF to transmit a digital 0. To transmit a digital 0 the laser is biased slightly above the threshold where the optical output power will be low. Conversely, to generate a digital 1, a current pulse is applied to the laser so that its optical output power will jump significantly as shown in figure 7.
The amplitude of the signal can be adjusted as not to exceed the maximum current rating of the laser but high enough so that the difference in optical power representing a 1 and that representing a 0 is easily distinguished by the optical receiver. The decision level of the receiver is usually set halfway between the minimum and maximum transmitted power. The ratio between the high power state (1) and the low power state (0) is called the extinction ratio. Maximizing the extinction ratio while not over driving the laser transmitter results in better noise immunity and better sensitivity at the receiver.
Laser Spectrum
Two laser types are commonly used in optical communication systems, the Fabry Perot laser (FP) and the Distributed Feed Back laser (DFB). An FP laser is also known as a multi-longitudinal laser because its laser cavity resonates at multiple regularly spaced wavelengths, Δλ=λ2/c. Figure 8a presents an FP laser’s spectral output, notice the discrete regularly spaced wavelengths emitted from the laser. The spectral width of an FP laser is measured at the –3dB point or the FWHM and is typically around 4nm. The FP laser’s greatest advantage is it’s relatively low cost and is used for multimode, point-to-point direct links and low bandwidth networks
Figure 8b is the spectral output of a DFB laser. Immediately it is obvious that a DFB laser’s output is spectrally narrower than its FP counterpart. Periodic changes in the laser’s refractive index make it highly reflective at certain wavelengths. Reflections provide feedback to the laser creating high losses at all but one wavelength so that the laser oscillates at one frequency only, making the DFB essentially a monochromatic source.
Evertz utilizes DFB lasers for all of its high bandwidth WDM systems. The spectral width of these lasers is 0.1nm at FWHM making DFB systems less susceptible to dispersion effects than those systems employing FP laser transmitters. The complex make up of DFB lasers increase the cost of the transmitters and should only be used when necessary.
Laser Back Reflection
All lasers are susceptible to back reflection. Back reflection or as it is sometimes called, optical return loss (ORL) is a peculiar phenomenon where by a fraction of the transmitted optical power will reflect back toward the source upon encountering variations in refractive index. Splices, patches and defects in the fiber all can cause back reflections. Fiber with more than 20dB of back reflection is considered quite high and optical isolators should be used on laser sources. For example the back reflection of an air-glass interface, as one would see in a broken fiber is -15 dB. If a laser with –5dBm output power was launched into this broken fiber, then the laser would have –20dBm optical power reflecting back into the laser cavity disrupting the standing optical wave generating noise in the output optical signal.
A laser’s susceptibility to back reflection is determined by its coupling efficiency. For example a laser that couples only 25% of its output power into the fiber then only 25% of the total reflected power would be coupled back into the laser cavity. Likewise, a laser with 80% coupling efficiency would have 80% of the reflected power couple into the laser cavity making it very sensitive to back reflections.
To combat the negative effects of back reflection Evertz laser transmitters are equipped with optical isolators limiting the amount of optical power allowed to re-enter the laser cavity. In systems with multiple fiber patches and splices reflections becomes a major concern for single fiber bi-directional transceivers. Using a two-fiber module with the transmitter of one card connected to the receiver of the other card will eliminate reflection issues allowing a much larger link budget.
