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Alluxa's new class of Infrared (IR) bandpass and dual band filters bring the precision of dielectric filters to the IR between 2 and 5 microns. They are manufactured with an advanced, plasma enhanced PVD process using durable, hard metal-oxide, and durable semiconductor front surface thin films. The films have excellent environmental stability. The OH (water) band at 2.7 microns shows loss below the measurement limit even after 10 days of harsh Mil-810G temperature and humidity cycling.
The narrowband and dual band IR filters are the latest innovation in the Ultra product line to come from Alluxa's novel plasma hard coating process. The filters will offer performance improvements and new design options in a wide range of applications from gas sensing, chemical monitoring, and night vision, to LIDAR and FTIR Spectroscopy.
Resonant cavity bandpass filters' basic design principles are well known and relatively simple. The filters are composed of stacked, resonant Fabry-Perot cavities where quarter-wave stack reflectors are spaced apart by a cavity layer. An excellent book describing the subject further is 'Thin Film Optical Filters' by Dr. Angus Macleod. Designing filters to have improved squareness, or steeper slopes, is a relatively straightforward matter of increasing the number of cavities in the filter design. To make a filter narrower, the designer increases the optical thickness of the spacer layer or increases the reflectivity of the quarter-wave stack mirrors. This is analogous to increasing the finesse in any other resonant cavity.
An alternative design approach commonly used in this industry is to combine a Long Wave Pass filter (LWP) with a Short Wave Pass filter (SWP). This design approach is favored for wider filters, and for filters which may have asymmetric slope requirements. Slopes can be increased by simply adding layers to the LWP or the SWP.
Adding cavities to the resonant cavity or layers to the edge filters challenges the deposition process control system, and invariably introduces undesirable ripple and loss to the pass band of the filter. Alluxa has created a new low noise IR optical monitoring system that provides the utmost in layer thickness control, which enables significantly more cavities and layers to be added to designs that are still practical to manufacture. The novel, advanced computer control system constantly measures the filter function and compensates for thickness errors associated
Traditional narrowband filters have traces of water incorporated in them during the deposition process, or adsorbed from the atmosphere post deposition due to the porous, low density film structure characteristic of low energy evaporated films.
Alluxa's new plasma deposition process incorporates essentially zero OH during deposition and the fully dense films have no porosity to adsorb water. As a demonstration, a fully blocked narrow bandpass filter was constructed with a wavelength centered in the water band, shown below in Figure 1. This filter is composed of a multi-cavity bandpass deposited on side 1 with a blocking filter that blocks from the UV to 6 microns on side 2. The substrate material is silicon. The filter is positioned to transmit at the center of the OH stretch band to demonstrate the low levels of OH bonds incorporated in the filter.
Optical filters that transmit two pass bands, or dual bandpass, offer different and complex challenges when manufactured. These can be built using resonant cavity techniques, but more often they are combinations of edge filters, notch filters, and/or wide band filters.
Figure 2 and Figure 3 below are two examples of fully blocked dual band filters on silicon substrates. The pass band center wavelengths, pass band widths, and the filter slopes are arbitrary and relatively easy for the designer to adjust.
A variety of filters including Anti-Reflection coatings (AR's), bandpass filters, and dual bandpass filters, all on silicon substrates, were subjected to harsh environmental stability testing. These filters all successfully passed 10 cycles of the MIL ' 810G, Section 507.4 Humidity Testing Procedure, generally considered the toughest environmental test for IR filters. The filters show no change in performance within normal measurement variation because of the dense film structure produced by Alluxa's proprietary energetic plasma deposition. All filters also passed the snap tape adhesion test.
Because there was no increase in loss or absorption in the transmission measurements, the filters continued to show low OH levels in the film, the results indicating no change within the measurement limit of the spectrophotometers. The results of the AR and dual bandpass measurements made before and after using an FTIR spectrophotometer are shown below.
Alluxa's introduction of IR bandpass and dual band filters are intended to deliver new high performance filter advantages to the IR spectrum between 2 microns and 5 microns. Using Alluxa's proprietary hard coating deposition process, these filters offer many improvements to designers working in the IR including very high transmission in the water bands between 2.5 microns and 3 microns, full MIL 810 Humidity acceptance, multi-band capability, and Alluxa's very high transmission and steep edge slopes.
Alluxa designs and manufactures next generation hard coated optical filters using a proprietary plasma deposition process in Santa Rosa, CA. Alluxa's unique, purpose-built deposition platform and control systems were designed, developed, and built by our team in Santa Rosa, CA to address the demanding requirements of the next generation of systems and instruments. This unique technology allows Alluxa to create the world's most challenging filters at breakthrough price points.
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In wavelength-division multiplexer (WDM) and passive optical network (PON) modular design, single band pass filters and multiple band pass filters are used for the same purpose: permitting narrow wavelength ranges to pass through while rejecting wavelengths outside that range (known as the filter's upper and lower cutoff frequencies).
Multiple band pass filters are used to transmit two or more standard coarse wavelength division multiplexing (CWDM) channels, separating them from the other CWDM bands ' replacing two or more single band pass filters with a single component.
This article details the advantages of using multi band filters in place of two or more single band filters, examining WDM module performance, reliability, cost, and dimension concerns in design and production. It also establishes Iridian Spectral Technologies' unique expertise as both a progenitor of multi band filter technology and a present-day leader in multi band filters' telecommunications applications.
Traditionally, single band pass filters have been used in telecommunications; one filter allows one wavelength range to pass while rejecting other nearby wavelengths. For example, if a single band filter's central wavelength is , the filter's permissible wavelength range spans .5 to .5 nanometers (nm).
The core telecommunication range generally starts around or nm and reaches between and nm, across which numerous single band pass filters may be applied. Specifically, ITU standard CWDM band pass filters pass a 15 nm range and reject 25 nm outside. ITU standards for CWDM stipulate a channel spacing of 20 nm each, meaning a designer can start with a central wavelength of nm and progress to , , , and so on.
In , Iridian and a communications system designer in the South Korean market first considered and integrated two band pass filters on a single filter chip. Now, for example, one filter chip could permit pass bands where one filter's central wavelength is nm and another filter's central wavelength is nm (in lieu of using two single band pass filters with the same central wavelengths). This capability has revolutionized WDM and PON module and system design by simplifying connections, conserving space within the module, and eliminating costs associated with sourcing and implementing numerous single band filters.
Iridian and its Korean partner had recognized the market need for such a component; the communications system designer needed technologies to enable and improve LTE (long-term evolution, also sometimes referred to as '4G') wireless transmission. Fast-forward to today, and market need for multi band pass filters is driven by the technology demands of fifth-generation (5G) wireless transmission (from the mid-wave band to the mmWave band), shrinking consumer devices, and increased connectivity in numerous technologies, including military, consumer, and medical devices.
The most popular application for multi band filters currently is wireless station interconnects for 4G, LTE, and 5G systems. Multi band filters also can be used in any CWDM system where minimizing component count and reducing component footprint is an advantage (i.e., GPON, XG-PON, XGS-PON, which leverage three specific wavelength channels: , , and ).
Band pass filters do not have stand-alone functionality; they must be packaged (the result being a component). Within the component are the filter itself, two collimating lenses, and a fiber lead out. Using two or more single band pass filters within a module, a designer must individually connect each fiber-connected component to one another.
Alternatively, a dual band fiber filter saves the designer at least one fiber connection, while a triple band filter combines three distinct filters into one component and saves at least two fiber connections. The advantage for the designers is a reduction in the overall footprint for the equipment 'closets' used to install wireless interconnect stations, which becomes more critical as the number of stations increases in 5G networks and the cost of the urban real estate (needed for these closets) soars.
The maximum density possible (i.e., maximum number of filters packaged/bands feasible in one component) depends on customer specification. While Iridian has achieved as many as nine bands within the range between nm and nm, dual or triple band filters are more typical in the telecommunications wavelength range.
Further, additional bands do not encourage any performance degradation; on the contrary, multi band pass filters are capable of producing a flat pass band and good reflectance isolation in almost all use cases. However, channel spacing must remain reasonable and dead bands must not be too 'steep.' For example, while 20 nm is an achievable band space, 50 or 60 nm between adjacent pass bands is preferable. Otherwise, the filter skews closer to a narrow notch filter.
Ultimately, system design is simplified, and system performance is improved, by using multi band pass filters. The designer reduces the number of components within the module, optimizing use of space/dimensions. Design is simplified by removing the number of fiber connections necessary, and removing these extraneous connections reduces insertion loss.
Despite these advantages, the expense of using multiple single band pass filters versus single multi band pass filters does not scale linearly with the number of bands. The principal deciding factor in cost scaling is steepness of the dead band, which determines design thickness and establishes how difficult (i.e., expensive) the design is to construct. Simply put, a design with 5 nm dead bands is going to be more expensive than something with 10 nm dead bands.
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