Yongbao Xin and Michelle Briere, Iridian Spectral Technologies
The dual trends of device miniaturization and rising demand for high-speed services are increasing the need for space-saving optical filters — a challenge that traditional multi-component filter designs are unable to meet. This is because the number of filters required to manage different functions and wavelengths hinders a compact design, making it impossible to build increasingly compact modules and devices.
Fortunately, multifunction filters for telecom applications are keeping pace with that demand, advancing in both capability and reliability. However, for telecom OEMs designing devices that feature all new specifications, push the limits of performance, and/or combine functionality in unique ways, close collaboration with a knowledgeable filter supplier is essential to unlock the full range of benefits multifunction filters can provide.
A World Of Multifunction Possibilities
The immediate benefit of multifunction optical filters is, of course, saving space within telecom device modules. By minimizing the number of components and connections, multifunction filters can reduce insertion loss throughout a module, optimizing its performance. The use of multifunction filters also minimizes a device’s inherent operational risk (i.e., by minimizing potential points of failure) and simplifies the module designer’s engineering process.
Iridian Spectral Technologies’ innovations in optical filter technology typically are driven by customer demands and market trends. Through 2025, we have seen increased customer interest in the following areas:
Hybrid Gain-Flattening Filters (GFFs) — A hybrid GFF combines the functionality of a gain-flattening filter (GFF) and a notch or a band pass filter (BPF) in one component. Iridian Spectral Technologies is experiencing increased interest specifically in GFF filters with laser line source blocking capability, as well as improved peak-to-peak error function (PPEF). A hybrid GFF blocks that laser line and flattens gain using a single component — functions that would otherwise require two separate filters, leading to twice the insertion loss.
Compensating GFFs — These GFFs compensate PPEF generated by one GFF through the addition of a second GFF downstream, or sometimes several, to achieve a low combined error function. Iridian Spectral Technologies is at the cutting edge of higher-volume, low-PPEF filters. We have developed our capability to mass-produce high-quality compensating GFFs with exceptional repeatability, helping to standardize them for the industry.
In addition to being the only optical filter supplier capable of such high volume, Iridian Spectral Technologies is breaking new ground in terms of deeper modulation depths for these filters. Specifically, we are the only supplier capable of achieving PPEF<0.1dB modulation depths up to ~6dB, as well as PPEF<0.4dB for modulation depths of 8dB or more.
Iridian Spectral Technologies has produced GFFs up to 16dB modulation depth, pushing the limits of PPEF, though some performance trade-offs become necessary as target curves go deeper. Most importantly, we can achieve these PPEF and modulation depth levels in multifunction filter designs.
Dual Band Pass Filters (DBPFs) with Integrated TAP Filters — TAP filters are critical for monitoring and analyzing network traffic, so integrating them into a multifunction DBPF offers wide-ranging impact. Iridian Spectral Technologies experienced some success in design relevant to this exciting new area of development, and we are excited to explore it further with innovative customers.
Dense Wavelength Division Multiplexing (DWDM) Filters Featuring Multiple ITU Channels — DWDM enables fiber-optic telecommunications networks to transmit signals of several wavelengths simultaneously; ITU channels refer to the wavelength spacing used in DWDM. Iridian Spectral Technologies groups several channels together before adding a skip channel (hence, the term “skip filters” also used for these components). While skip filters can be constructed with single pass-through channels and skip channels, multiple-channel grouping filters are increasingly desirable as their performance and reliability is established.
Another notable trend in optical filtration relevant to elecom is a shift toward OEMs replacing Fiber Bragg Grating (FBG) technology with optical filters. Notably, however, thermal management in the passive optical filters used in telecom is not as critical as in datacom, where temperature control is crucial to protecting densely packed electronic components. Nonetheless, thermal drift in Iridian Spectral Technologies’ telecom filters is minimal, e.g., both the blocking function and the flattening function in a hybrid GFF remain within spec during temperature shifts.
Combining Filter Functions Is A Team Effort
To get the most out of multifunction filters, it is important that telecom device OEMs work with their filter supplier to clearly communicate performance specifications and package size needs. Working closely with a filter supplier helps the OEM understand not only the possibilities of filter performance in their package design but also any potential limitations. This collaboration also enables the filter supplier to assist the OEM in appropriately tuning and packaging the filters, ensuring the OEM is able to receive each component’s maximum benefit.
Switching from a design that utilizes single-function filters to one that combines functions is likely to result in minor temperature differences and the need to tune the hybrid style filter for optimal performance. Importantly, Iridian Spectral Technologies can provide simulation data on each filter during the design phase, before a physical component even has been produced. Performance specifications including signal-to-noise (SR) ratios and angle of incidence (AOI) are part of the simulation, enabling customers to clearly understand how hybrid filters will perform within their packaging versus non hybrid filters. In many cases, simulation data can be generated within a few hours of the customer request.
Iridian Spectral Technologies provides a design guarantee that simulation data will accurately reflect the performance and tuning of the manufactured component. This ensures customers can have confidence that the results they see in our simulations will be reflected in the physical product. Most customers use our simulation data or even raw measurement data as a guide to achieve peak performance or the AOI they want to utilize. However, they sometimes lack the expertise to tune each filter to a level that allows them to take full advantage of its capabilities.
Without our data, customers implementing high-precision components often incur heavy costs in labor, filter tuning, and optimizing their production lines. Even though multi-band filters and multifunction filters are not common off-the-shelf components, Iridian Spectral Technologies can mass produce them at high volumes and at prices similar to single-band and single-function filters.
We constantly work to improve our products and their performance while helping our customers to accomplish the same. In a transactional relationship, this dynamic can create a rift between the OEM’s engineering team, focused on design, and the personnel responsible for managing costs. Yet, under the partnerships we strive to build, the customer’s internal groups can better align on spending and design goals, ultimately reducing their overall costs through higher quality, higher functioning components. Innovative high-performance optical filters are Iridian’s sole focus; accordingly, our success is directly measured in terms of our customers’ success.
Iridian Spectral Technologies strives to show customers where our capability overlaps with their needs. We design and build multi-band and multifunction optical filters that are vital to telecom module miniaturization without compromising performance. This combination of space-saving and high performance frees telecom module designers to innovate more freely and affordably, supported by Iridian Spectral Technologies through design support, prototype evaluation, physical samples, and expert modeling simulation data.
