Types of Optics Spectrum Analyzers

Dense wavelength division multiplexing (DWDM) is the hottest technology in fiber-optic networks today. It enables high-speed data transmissions over fiber.

Test equipment manufacturers are responding by developing new types of testers—optical spectrum analyzers (osas) and multiwavelength meters (mwms). These instruments have different strengths that must be evaluated before choosing the right one for your DWDM test needs.

Diffraction Grating Method

The diffraction grating method (DGM) is a technique that uses a diffraction grating to measure strain, which allows for strain resolutions down to 1 u [29]. DGM is also able to perform a full strain analysis on a specimen, which is beneficial for mechanical testing.

A diffraction grating is a screen that has been fitted with a series of elongated openings or slits that are uniformly spaced and are parallel to the main plane. The individual wavefronts that emerge through the slits constructively interfere, resulting in a diffraction pattern with peak intensities that occur at certain angles relative to the plane of the screen and the angle of the incident light wave.

Gratings can be made from a variety of materials and can be used in a transmissive or reflective mode. Gratings with groove spacings that are less than half the wavelength of light are called subwavelength gratings and exhibit special optical properties.

Diffraction gratings can be optimized for specific wavelengths by adjusting the angle of the blazed beam and the density of the grooves. This optimization may involve varying the profile of the groove, its depth, and the facet angles.

Some diffraction gratings are designed to operate at or near the Littrow condition, which is the optimal position for maximum diffraction efficiency. This is because the direction of each output beam is dependent on the wavelength of the incident laser beam.

The intensity of the light is measured using a beam profiling camera, and the diffraction patterns are normalized by scaling the average peak irradiances to unity. Differences between the maximum, minimum, and mean scaled irradiances are calculated for each of the 20 measurements on the fabricated grating, which is shown in Fig. 2.

The dfab performance is calculated by comparing the diffraction pattern of the grating to that of the designed dfab, and then accounting for fabrication errors. The variances for the fabricated gratings after scaling to the designed E/N and accounting for fabrication errors are 0.156 +- 0.027, 0.195 +- 0.031, and 0.201 +- 0.047 for gratings 1-3, respectively.

Fabry-Perot Interferometer Method

The Fabry-Perot interferometer is an effective method for measuring a wide range of radiation, including light in the visible, UV, and near-infrared wavelengths. This method is particularly useful for applications that require small-sized, low-cost interferometers. It can be made manually or by an automated production line, and the required materials are inexpensive.

The heart of the Fabry-Perot interferometer system is a pair of glass optical flats with parallel, reflective surfaces. These flats are spaced micrometers to centimeters apart. They are usually wedged to prevent ghost fringes from appearing on the rear side of the flats. They are often coated with an anti-reflective coating to make them more suited for interferometry.

This type of FPI is particularly useful for spectrochemical analysis, since it can measure different types of samples simultaneously and at different spatial locations. It can also be used to measure particles of varying size and shape, since the Fabry-Perot cavity can accommodate a very large number of discrete mirror positions.

Finesse, a measure of the quality of a Fabry-Perot mirror, is a good indicator of its reflectivity. High finesse osa dwdm means that a given wavelength of light is reflected by a much larger portion of the mirror than it would be with a lower reflectivity.

Typical Fabry-Perot mirrors have reflectivities of around 4% for bare glass and up to 95% when coated. Using state-of-the-art technology, current FPI systems can provide finesse factors of up to thousands.

For example, the following animation shows false color transients for a silicon (n = 3.4) Fabry-Perot etalon at normal incidence:

A high-finesse etalon (red line) has sharper peaks and lower transmission minima than a low-finesse etalon (blue). The higher the finesse factor, the better the performance.

In order to produce a Fabry-Perot interferometer that is controllable with one or several actuators, the width of the gap between the mirrors must be controlled. This is not an easy task, because bending of the mirrors must be reduced and/or avoided.

This can be done by placing an intermediate structure between the mirrors, as described above. The width of the gap can then be measured and adjusted during the assembly process, making it possible to achieve a much more precise control over the gap during calibration and use. This means that the interferometer can be produced with significant advantages over prior art technology, such as a wider pass band, more accurate parallelism of the mirrors and avoidance of substantial offset voltages in the control of the actuators.

HeNe Laser Method

A HeNe laser is a glass tube filled with 85-90% helium gas and 10-15% neon gas, which is the actual lasing medium. Its primary output wavelength is 632.8 nm (usually simplified to 633 nm) and it has good power stability, narrow beam profile, low divergence, and output coherence.

HeNe lasers are used in a variety of applications. They are often used for spectroscopy, dwdm, and other advanced signal analysis applications that require high resolution bandwidth. They are also common in fiber-optic network monitoring and other telecommunications applications, where they can help ensure that all channels are functioning properly.

These lasers can be used to calibrate a grating-based optical spectrum analyzer (osa). For dwdm applications, this method is particularly useful because it offers wavelength accuracy measured to as low as 0.001 nm. This is critical to ensuring system robustness when worst-case conditions of optical component tolerances converge.

Another benefit of this dwdm method is that it provides wavelength stability for long distances, which can be a challenge in some applications. This is especially true for dwdm applications that are prone to transient losses in the input fiber-optic cable.

In addition, HeNe lasers provide excellent beam quality and angular alignment, which is essential in precision instrumentation and measurement applications. This is important because it can affect the accuracy of your measurements.

The beam quality of a HeNe laser is dependent on several factors, including the length of the gas tube and its reflectivity. The longer the tube, the better it is at achieving Gaussian beam profiles and maintaining good angular and spatial alignment with respect to the laser housing.

HeNe lasers are also typically very low-cost, so they can be a great option for a wide range of applications. However, these lasers have a few drawbacks that might prevent them from being a viable alternative for certain industrial applications.

One of the most important drawbacks is that HeNe lasers have short life spans, which are commonly less than a year for 24/7 operation. This is a major concern for many industrial applications that require a long life span and reliability.

Wavelength Resolution

Wavelength resolution is one of the most important characteristics osa dwdm that set apart an optical spectrum analyzer (OSA) from other types of spectrometers. It allows a user to accurately determine the wavelength of an optical signal by sweeping a narrowband filter over the spectrum. This type of OSA is especially useful in dense wavelength division multiplexing (DWDM) applications, where its ability to distinguish between closely packed wavelengths is critical for the accurate measurement of optical channel power and optical rejection ratio (OSNR).

A common way of calculating resolution is to apply the Abbe diffraction limit or the Rayleigh criterion. These theories define the theoretical limits of resolution in a diffraction-limited system, where two points of light can be distinguished from each other by slicing the spectrum by a narrow band filter.

Generally speaking, higher numerical aperture values will produce higher degrees of resolution. In addition, there are several other factors that influence resolution, including the diffraction gratings surface density and the distance between the grating and the detector.

As an example, a commercial scanning FPI can easily provide spectral resolution better than 10 MHz, which is approximately 0.08 nm in a 1550 nm wavelength window. However, a narrower wavelength window may require a smaller diffraction grating.

Another important factor is the optical noise level, also known as the optical rejection ratio or ORR. High ORR values are essential for obtaining an accurate OSNR measurement. A good ORR gives a clean noise floor near the center wave length of the channel under test, and allows a spectrometer to distinguish between closely packed channels that are close together, says Moench.

The ORR is a vital parameter for DWDM tests, since it describes how robust the channel can be when it overlaps its weak neighboring channels. Hence, a low orr means that a signal can easily overshadow its weak neighbors.

For a good level of accuracy, an OSA should have an ORR value that is at least 50 GHz away from the center wavelength. This will give the noise floor a sufficient amount of time to separate a signal from its neighbors, so that the signal is not smothered by other signals. This is especially important in a dense wavelength division multiplexing (DWDM) test application, where a smothered signal will result in an inaccurate snr measurement.