Optical Spectrum Analyzers (OSAs) and Dense Wavelength Division Multiplexing (DWDM) Technology

The growth in cloud computing, Internet of Things (IoT) devices and access to mobile broadband has created a demand for large-capacity optical backhaul networks. Dense wavelength division multiplexing (DWDM) systems have emerged as a means of transmitting signals across long distances with high reliability.

DWDM systems are complex, and test and measurement is vital to ensure that they function properly. Optical spectrum analyzers (osas) and multiwavelength meters (mwms) are two types of optical testers that can be used to characterize dwdm components and systems.

DWDM technology

Dense wavelength-division multiplexing (DWDM) technology is an innovative way of using a single fiber to support several data channels. DWDM can be used to increase the bandwidth and capacity of an existing fiber infrastructure or to expand the reach of a new network.

DWDM uses tightly spaced wavelengths in the C-band (1525 to 1570 nm) and L-band (1565 to 1620 nm) to fit more channels on one fiber pair. This tighter wavelength spacing allows DWDM to carry more data per fiber pair at higher data rates, resulting in a lower cost of ownership for networks.

However, DWDM technology isn’t without its limitations, as it’s also not immune to nonlinear effects in transmission. Hence, DWDM systems must be tested to ensure that the BER is appropriate for their performance and interference suppression is achieved.

To test DWDM networks, a range of telecommunication testing instruments are available to meet the unique challenges that this technology presents. These include optical spectrum analyzers (osas) and multiwavelength meters (mwms).

An osa is an analytical instrument that divides an input signal into constituent wavelengths, then measures the power of each channel in each of those wavelengths. Various types of osas are available, including those with diffraction gratings and tunable filters.

A grating-based OSA is ideal for many dwdm applications, since it can measure the power of individual channels with high resolution and dynamic range. Its rugged design makes it a good choice for field use.

The Fabry-Perot interferometer method is another type of osa that is particularly well suited to dwdm testing, since it has no moving parts and can detect closely spaced channels. This technique is also very accurate, although the dynamic range is limited.

Another commonly used osa is the Michelson interferometer method, which uses parallel mirrors to create a resonant cavity for filtering. This method provides high spectral resolution, although it has a low sensitivity to shock and has poor noise immunity.

DWDM test equipment must be able to handle the rigors of a busy laboratory and also perform tests in a field environment. Consequently, portable units must offer automatic pass/fail testing routines and increased wavelength testing capabilities (e.g. 10-GHz channel spacing over 800 channels) and special testing software for DWDM components.

Optical spectrum analyzers (osas)

Optical spectrum analyzers (osas) measure the power distribution of light waves over a specified wavelength range. They are an essential piece of testing equipment for telecommunications because they can accurately characterize light sources, perform WDM network analysis, optical amplifier assessment, and OSNR measurement.

Typically, an OSA has a broad wavelength range and can measure multiple signals at once in dense wavelength division multiplexing (DWDM) systems. When a signal is read by an OSA, it is divided into its constituent wavelengths, and the profile of each signal is displayed graphically with power on the vertical axis and wavelength on the horizontal axis. This allows per-channel analysis of the light signal and its spectral interaction with other signals on the same fiber.

To accomplish this, an OSA uses a monochromator with diffraction gratings to separate the different wavelengths of light and allow only specific ones to reach its photodetector. This enables high resolution within the frequency range of 100 MHz to 10 GHz, which is sufficient for measuring laser chirps.

Another popular type of OSA is a scanning Fabry-Perot interferometer (FPI). This design can provide high spectral resolution with a lower resolving power, and it can be used to detect the chirps of lasers.

A third type of OSA is a hybrid design, such as the Littman monochromator, that combines diffraction gratings with a osa dwdm multi-pass grating. The combination of these two technologies enables a high resolution OSA with a low operating temperature.

The ability to measure a wide variety of wavelengths and power levels has increased the demand for optical spectrum analyzers. These instruments are used for testing and characterization of fiber optic networks, including CATV HFC/DAA, 5G x-haul, and hyperscale data center interconnects.

In these networks, light pulses transmit digital information over thousands of kilometers. As these networks have evolved, so have test solutions and practices. The demand for more accurate, compact, and cost-effective test instruments has driven the development of optical spectrum analyzers.

An OSA can be configured with various features to meet the specific application requirements of a particular network. Typical features include higher acquisition and measurement speed, better resolution, greater sensitivity, and wider bandwidth. In addition, the averaging of detector signals over a longer time reduces the effect of noise and allows for improved accuracy.

Multiwavelength meters (mwms)

Dense wavelength-division multiplexing (DWDM) technology carries several different light signals osa dwdm at once on an optical fiber. The resulting higher bandwidth offers unprecedented capacity for information transmission, but it also presents test and measurement challenges. A new generation of equipment that is sensitive to wavelength is needed to help solve these problems.

Optical spectrum analyzers (osas) and multiwavelength meters (mwms) are two types of test instruments that use interferometry to divide a light signal into its constituent wavelengths. Once these interference patterns have been treated via a fast Fourier transform (fft), they can be displayed in a graphic format for interpretation.

To achieve its high-performance accuracy and resolution, an mwm typically incorporates a scanning Michelson interferometer and a helium-neon (HeNe) laser for reference measurements. “This is a significant advantage over an osa with no reference, as the HeNe laser provides excellent absolute wavelength accuracy,” says Brian Samoriski, president of Bristol Instruments (Victor, NY).

Another multiwavelength meter uses a diffraction grating with tunable filters to create its interference patterns. These diffraction gratings have three parameters that determine their resolution: the density of the lines on their surfaces, the distance between them, and the configuration of their optical circuits. These three factors combined influence the unit’s wavelength resolution, a few thousandths of a nanometer at best.

Although both osas and mwms can measure a wide range of power levels, osas have better absolute power accuracy. They are especially useful for crosstalk tests, where the unit’s ability to detect noise between channels makes it a good choice.

In a dwdm system, the ability to measure small drifts in channel wavelengths over time is crucial. mwms have much better orr, or optical rejection ratio, than osas, which means they are less susceptible to errors caused by dwdms poor channel spacing.

Mwms can have a wide dynamic range, which is important for measuring dwdms snr (signal-to-noise ratio). They can also perform multiple tests over time to measure dwdms drift in channel wavelengths.

In addition to its ability to measure dwdm’s snr, a multiwavelength meter can also be used for polarization management. It can differentiate between asymmetric transmissions and symmetric transmissions by analyzing polarization patterns. This can help minimize the effects of dwdm’s polarization diversity, says John Gariepy, product line manager at JDSU Fiberoptics Division (Germantown, MD).

Applications

DWDM technology is a powerful technique that allows dense wavelength division multiplexing of multiple signals over a single optical fiber. The system multiplies the information-carrying capacity of the fiber by the number of different wavelengths it carries, thereby increasing bandwidth. It is used in many telecom applications including telecommunications, data centers, video conferencing, and remote monitoring.

Optical spectrum analyzers (osas) are a type of dwdm test instrument that can divide an optical signal into its constituent wavelengths and measure the power of each. They are commonly used to monitor and measure the power levels of individual DWDM channels, as well as to determine how each channel changes over time.

There are several osa models available, each with its own set of features. Some are designed to specifically handle dwdm networks, while others can be used for other types of testing.

A common feature in most osas is the ability to separate and display channel wavelengths. This is especially useful for troubleshooting and determining the quality of an optical link.

The accuracy and resolution of an osa depend on three parameters: the density of lines on the diffraction grating, the distance between the grating and detector, and the configuration of the optical circuit. The combination of these factors determines the osas wavelength resolution, which can be as low as a few thousandths of a nanometer.

Another important feature of an osa is its optical rejection ratio, or orr. This is how accurately the osa can separate high-power channels from each other, and how robust its inter-channel noise is. Orr can also be improved by using a double-pass osa, which further divides the signal into its components.

However, a disadvantage of the double-pass design is that it can be very large and fragile. This makes it difficult to transport and use in the field.

For CWDM networks, a more compact and less expensive OSA is typically necessary. The RXT-4500 full band OSA option is an ideal choice for determining the presence and location of each of the 16 DWDM wavelengths, as well as checking their power levels.