Optical spectrum analyzer

Friday, 12 January 2007

An optical spectrum analyzer (OSA, spectrometer) is an instrument that measures the optical power as a function of wavelength or frequency. The growth in telecommunication traffic is driving the development of technology to ever expand the available bandwidth of network communication. Optical communication systems are a substantial and fast-growing constituent of modern communications networks. These systems demonstrate greater bandwidth, lower power requirements, and lower-bit error rates than conventional, non-optical transmission systems. Because of their superior performance, optical systems are highly desirable have shown that they are better suited to high-speed data, video, voice, and other integrated transmissions. In fiber optic systems, a convenient way to carry multiple channels over a single fiber without interference between the channels, for both one-way and bi-directional communications, is by means of a technique known as wavelength division multiplexing (WDM). The advantage of WDM systems is that the transmission capacity of a single fiber can be increased. In the WDM (Wavelength Division Multiplex) optical transmission system, a plurality of signal channels having different wavelengths are transmitted simultaneously through an optical fiber. Therefore, the WDM optical transmission system is advantageous in that transmission capacity per optical fiber can be increased by increasing the number of wavelengths, saving the number of optical fibers. The WDM optical transmission system mainly consists of an optical transmitter provided at the transmitting end of an optical transmission line, an optical receiver provided at the receiving end thereof and the optical transmission line between the optical transmitter and the optical receiver. The optical transmission line usually consists of a plurality of optical amplifiers for amplifying WDM light signal and a plurality of optical fibers for connecting the optical amplifiers mutually. WDM systems typically comprise multiple separately modulated laser diodes at the transmitter. These laser diodes are tuned to operate at different wavelengths. In a WDM system, a plurality of optical channels are carried over a single optical fiber. This is made possible by assigning each channel a different optical wavelength; each of the several wavelengths being transmitted within the optical fiber simultaneously. When combined in an optical fiber, the WDM optical signal comprises a corresponding number of spectrally separated channels. Along the transmission link, the channels are typically collectively amplified in gain fiber, such as erbium-doped fiber and/or regular fiber, in a Raman pumping scheme. At the receiving end, the channels are usually separated from each other using thin film filter systems, for example, to thereby enable detection by separate photodiodes. In WDM systems, a multiplexer is operative to receive a plurality of data channels each having a separate wavelength and to combine and transmit the data channels to a single optical fiber. At the receiving end of the fiber, a multiplexer can be used in reverse to demultiplex, or separate, the data channels according to their respective wavelengths, so that the data corresponding to each wavelength channel can be guided to its own detector or optical fiber.

In a typical fiber optic transmission network such as a wavelength division multiplexed (WDM) network information is transported between optical terminals by optical fiber links characterized with optical channels operating at distinct wavelengths. Optical fibers can be used to transmit or process light in a variety of applications, including delivering light to or receiving light from integrated optical components or devices formed on substrates, transmitting information channels in WDM optical communication devices and systems, forming fiber optic switch matrix devices or fiber array to array connector, and producing optical gain for optical amplification or laser oscillation. Signal quality of an optical signal transmitted from the optical transmitter degrades while traversing through the optical transmission line. The use of optical fibers to carry information substantially increases the distance separating optical network terminals. However, standard optical terminal interconnections are nevertheless limited by a number of factors including the optical power that can be launched into the interconnecting fibers, fiber loss, fiber dispersion and the sensitivity of optical receivers used in the optical terminals. As the optical signal is transmitted across the fiber optic communication system, the gain differences on a channel-by-channel basis accumulate. These gain differences can cause distortions of the optical signal shape and therefore lead to performance degradation. In most systems, the performance of this transit equipment must be monitored so that faults in operation of the optical transmission network can be isolated. Typically, the optical channel signal is traveling in the fiber are optically separated and their respective optical intensity is measured to determine the amount of light propagating in each channel. Current WDM communication systems strive for maximum transmission capacity by spacing optical channels as closely as possible, typically less than a nanometer (nm). As the channel spacing, decreases, monitoring the spectral characteristics of the channels becomes more critical in verifying system functionality, identifying performance drift, and isolating system faults. By measuring the optical intensity of each channel signal, several channel transmission parameters can be monitored including the optical channel signal power, the channel signal-to-noise ratio (SNR) and the amplified spontaneous emissions (ASE) present in each channel. Optical spectrum analyzers have many important applications for monitoring the spectra of optical signals. Optical spectrum analyzers are used for analyzing the output light beams from lasers, light emitting diodes (LED's) and other light sources. Optical spectrum analyzers are particularly useful for analyzing light sources for optical telecommunication, where it is preferable to insure that the optical carrier includes only a single, spectrally pure wavelength. With the development of high density wavelength division multiplexing (HDWDM) applications, for example as used in communications conducted via optical fiber transmission mediums, the need for quality optical spectrum receivers and analyzers has become acute. In particular, there is a demand for optical spectrum analyzers that are robust, compact and yet display a sufficient optical rejection ratio (ORR) close in to a spectral feature to be measured. An optical spectrum analyzer spectrally divides the light under measurement by transmitting the components thereof at different, wavelength-by-wavelength angles using a chromatic dispersion device, and detects the light thus spectrally divided by the chromatic dispersion device, using an optical detector. Advantages of optical spectrum analyzers are their dynamic range and performing measurements involving many discrete spectral lines. In optical spectrum analyzers, the light intensity of a light beam is displayed as a function of wavelength over a predetermined wavelength range. Parameters of importance include wavelength range, wavelength and amplitude accuracy, sensitivity, resolution, measurement speed, polarization insensitivity and dynamic range. Some optical spectrum analyzers are capable of scanning through a series of channels and analyzing a single channel or more than one channel at a time. Some spectrum analyzers can be configured to monitor networks of channels and provide information about a wavelength of one or more channels or can be configured to provide information regarding the presence or absence of particular channels. In general, optical spectra are measured using either a tunable filter or a spatially dispersing element where the deflection angle is a function of the wavelength. The dispersing element is in general a prism or a grating such that for each wavelength a certain angle of deflection is defined. Both possibilities require mechanical tuning of the element or a detector array for analyzing the optical spectrum. Dispersion in optical materials can be transformed into a wavelength dependent spatial intensity distribution. Another consequence of dispersion is a wavelength dependent distribution of the transmission times through the dispersive media, the so called chromatic dispersion. Critical performance parameters of optical spectrum analyzers, such as signal selectivity and measurement sensitivity, depend on the characteristics of optical filters within the OSAs. Optical filters having narrow bandwidth, low insertion loss and wide tuning range are advantageous, enabling OSAs to test complex optical signals, such as those within dense wavelength division multiplexed (DWDM) optical telecommunication systems.

Optical spectrum analyzers have been developed using a broad variety of technologies, including diffraction gratings, interference filters, Mach-Zehnder interferometers, Fabry-Perot etalons, birefringent elements, and prism configurations. As one of the optical spectrum analyzers which have widely been used in practice, the Czerny-Turner type optical spectrum analyzer is constituted to rotate a diffraction grating for diffracting light which is to be analyzed and which is irradiated to the diffraction grating, the diffracted light is thereby moved on a slit, and the optical spectrum is analyzed on the basis of the angle of rotation of the diffraction grating at the time of detection of the diffracted light across the slit. The optical spectrum analyzer of this type can analyze the optical spectrum at a high resolution. However, the optical spectrum analyzer of the aforesaid type is large and heavy. Therefore, it cannot be easily processed and is not suitable as a portable device. The Michelson interferometer technology provides high wavelength selection precision and good spectral resolution, but the robustness remains a challenge and optical dynamic range limitations of such instruments continue to present problems. Most OSAs use a wavelength tunable optical filter, such as a Fabry-Perot interferometer or diffraction grating, to resolve the individual spectral components. In the latter case, light is reflected off the diffraction grating at an angle proportional to the wavelength. Fixed fiber Fabry-Perot (fixed FFP) filters can be used as accurate wavelength references for the calibration of optical spectrum analyzers (OSA) to increase both accuracy and resolution of measurements. Fiber Fabry-Perot tunable filters have been successful commercialization for use in he first wavelength detection multiplexing (WDM) systems and have demonstrated robust and field-worthy operation. Although fixed FFP filters produce multiple, very accurately spaced, wavelengths, a consistent problem has been the difficulty of accurately identifying an individual wavelength among the multiple wavelengths produced. Diffraction grating technology is the most widely used for optical spectrum analyzers in fiber testing equipment. Diffraction gratings can disperse incident light with high resolution, so that in the grating output the diffracted angle is a function of wavelength. When a beam that comprises a plurality of wavelengths incident to a diffraction grating, the beam is diffracted into sub-beams that can be focused by a lens onto a set or an array of detectors that receive sub-beams having different wavelengths. Diffraction gratings are widely used to disperse light into its spectral components for measurement of the spectral content of an optical beam or signal. In many cases it is desirable or necessary that the grating provide a maximum amount of dispersion; this can be accomplished by using a grating that has a high groove density. High-resolution optical spectrometers are used to observe spectral features of an unknown signal. Some high-resolution optical spectrometers implement a heterodyne architecture, based upon principles of coherent optical spectral analysis, to achieve very fine measurement resolution. Spectrum analyzers can be classified into one of two different types: non-real time and real time instruments. Non-real time instruments customarily employ a sweepable frequency filter or local oscillator which is scanned through the frequency band of the signal being analyzed. Real time spectrum analyzers simultaneously monitor the entire signal band of the signal, usually by means of a bank of parallel frequency filters. Current state of the art real time spectrum analyzers employ digital signal processing techniques, which sample and quantize the signal in the time domain. The resulting digitized signal is then processed by means of either a Fast Fourier Transform (FFT) filter or a Finite Impulse Response (FIR) filter to obtain a frequency response output.

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