|Optical power meter|
|Wednesday, 22 November 2006|
In optical communications systems, it is very critical to maintain optical signal levels at their correct power settings. Failure to properly maintain a link can result in severed communications as well as data disruptions since both voice and data may be carried on the same fiber. Optical signals are used for a variety of purposes. For example, optical signals may be used in the field of communications, the signals being transmitted via optical fibers and various other optical equipment, such as switching equipment, multiplexers, and amplifiers. Optical signals also are used in measurement systems, detector systems, and the like. An optical fiber transmission system includes a plurality of individual transmission paths, each of which is constituted by a plurality of optical fibers connected together. Problems arise with the maintenance of such a system, particularly when a given transmission path has an unacceptable power loss. In particular in wavelength division systems (WDM), many impairments can cause some channels to have power levels that are too high or low. In long distance WDM systems in which optical signals generate nonlinear effects such as self-phase or cross-phase modulation, it is desirable to ensure that the power levels in each single span of the system remains well-equalized. Data is encoded in light pulses which are transmitted through the fiber optic cable. These lightwaves reflect within the fiber optic itself because of the difference in refractive indices of the fiber optic and the surrounding cladding. The cladding is a lossy glass that surrounds the conducting core. Because of this arrangement, the fiber optic cable is sensitive to slight perturbations to the cable, including mechanical stresses such as bending, twisting and pinching of the fiber itself. These perturbations cause the light in the fiber to be dispersed into the cable cladding, within a short distance, thereby increasing the bit error rate (BER) of the system. There is a need to test the various optical fibers and other equipment. It may be necessary to measure the power transmission in a large number of the optical fibers constituting that transmission path. The proper utilization of fiber optic systems requires precise instruments for measuring optical power and testing the integrity of optical circuits. Fiber optic test instruments are used to characterize the quality of fiber optic links. Often the testing involves the transmitting, reflecting or absorbing of light at one or more wavelengths and measuring light at one or more wavelengths. The optical power levels are monitored by optical instruments such as optical spectrum monitors (or analyzers) and optical power meters.
An optical power meter measures the light intensity of the input optical signal received by the selected output waveguide. The optical power meter may be used to monitor the power of the laser generating the optical signal, to measure the loss through the transmission medium, to test the receiving electronics, etc. Generally, optical power meters comprise a photodiode which outputs an electrical current proportional to the optical power of an input optical beam, an I/V amplifier which converts the output current of this photodiode to a voltage, a variable gain amplifier which amplifies the output voltage of this I/V amplifier, and an A/D converter which converts the output voltage of this variable gain amplifier to a digital signal. Solid state photodiodes permits accurate locating of the optical beam. They are also used because the optical bandwidth is in the region of most popularly used fiber optics. Some optical power meters contain both a source of optical power and a sensor for sensing optical power. An optical power meter measures the optical characteristics of a light emission element and the transmission characteristics of the optical signal in an optical fiber. A photoelectric conversion element such as a photodiode and a photo-transistor is provided as a photo detector of the optical power meter. Optical power meters generally do not discriminate wavelengths, rather they measure light intensity or optical power independently or substantially independently of wavelength. Some measurement system and other optical measurement systems use a tunable laser system to supply light in the visible or other wavelength range to a device under test (DUT), such as an optical fiber and/or other optical equipment. The output from the DUT may be measured using one or more optical power meters (OPM). Typically, to monitor the optical power, the optical fiber is detached from the photodetector associated with the transceiver and the impinging light is attached directly to a separate optical power meter. Then to use the link to receive data again, the fiber is reattached to the optical fiber link.