Optical system performance for a wide range of applications such as instrumentation, laser cleanup, telecommunications, and LIDAR are improved by thin film optical filters with high transmission, ultra-narrow bandwidth, and flat spectral profiles.
Alluxa’s new line of ultra-narrow filters are designed for such applications. These filters offer the narrowest and “squarest” filter profiles in the NIR and visible as well as transmission levels that approach 100%. Alluxa’s proprietary advanced plasma enhanced PVD process is used to manufacture these filters using durable, hard metal-oxide, front surface thin films. The ultra-narrow filters are almost impervious to environmental effects.
Basic Bandpass Theory
The design principles of resonant cavity thin film optical bandpass filters are well recognized, and rely on stacked Fabry-Perot resonant cavities. Fabry-Perot resonant thin film optical cavities are a pair of dielectric reflectors that are divided by a multiple halfwave of optical thickness or a spacer layer of a halfwave thickness.
A cavity can be between 3 and 50 layers based on bandwidth. “Thin Film Optical Filters” by Dr. Angus Macleod is an excellent book that further describes the subject. Designing filters to have improved squareness, or steeper slopes, is a relatively simple matter of adding more cavities to the filter design (Figure 1).
To make a filter wider or narrower, the designer either decreases or increases the thickness of the spacer layer, or decreases or increases the reflectivity of the dielectric mirrors.
Figure 1. Filter squareness is a direct function of number of cavities count.
The addition of cavities challenges the deposition process control system and invariably introduces unnecessary ripple and loss to the passband. Alluxa has developed a new low noise monitoring system that provides the ultimate in layer thickness accuracy and makes it possible to add the cavities almost without limit.
The novel, advanced computer control system constantly measures the filter function and compensates for thickness errors associated with prior layers. The advanced monitoring system produces filters with extraordinarily low ripple and steep slopes that consistently match theory.
The “Ultra” Class of Flat Top Narrow Band Filters
In the 1990s, flat top, ultra-narrow hard coated bandpass filters were pioneered by the telecommunications industry and achieved remarkably high and narrow performance results. The narrowest published result was “25 GHz thin film optical filter” presented at OFC in 2002 by Scobey and Fortenberry and demonstrated a bandwidth of 0.1 nm at 1550 nm.
However, filters this narrow suffered from severe phase delay effects (chromatic dispersion) across the passband due to the high resonant lifetime and were not useful for real world telecommunication systems. These filters were also extremely small, on the order of a few mm in size, so they were limited usefulness to instrumentation designers.
Narrow band filters in the visible region with useful diameters of 25 mm to 50 mm or larger have been traditionally limited to between 2 and 5 nm in bandwidth due to uniformity problems, control system limitations, and optical loss.
A new class of ultra-narrow bandpass filters developed by Alluxa solves these manufacturing problems and produces bandwidths of less than 1 nm in the visible and near IR with square filters and flat top shapes. Figure 2 shows the measured result of a completely blocked 532 bandpass filter.
Special spectrophotometer setups are needed to measure these narrow filters. For more information, refer to Alluxa’s related paper New Metrology Techniques for Advanced Thin Film Optical Filters.
Figure 2. Measured Transmission of a fully blocked 3 cavity flat top bandpass filter at 532 nm with 0.92 nm bandwidth and T>92%.
Filters that are 12.5 mm to 50 mm in diameter are used by the bulk of narrowband applications. For the first time, Alluxa has developed flat top ultra-narrowband filters by controlling uniformity of physical and optical thickness.
These filters are economically priced in formats up to 250 mm. Figure 3 displays the measured result of different radii of a 250 mm flat top bandpass on a 250 mm wafer designed to transmit the 532 nm laser line.
Figure 3. Measured results of 250 mm diameter, fully blocked 3 cavity flat top bandpass filter with 0.94 nm bandwidth with T>90%.
Shift with Angle
The filter function of a thin film filter built of resonant cavities shifts as it is tilted away from normal to shorter wavelengths. The shape itself remains almost the same until relatively high angles where polarization effects begin to become a factor.
It is well-known that this relative shift in wavelength changes proportionally to the square of the angle of tilt, for tilts up to a few tens of degrees. The proportionality constant is the inverse of the square of the effective index times two, where the effective index is a property of the cavities design and also the coating materials used. This effect is demonstrated in the angle measurements of the narrowband filter (Figure 4).
The user should consider the angle shift effect when determining the Field of View (FOV) requirements of the filter. The filter bandwidth may need to be increased for larger FOV applications.
Figure 4. Shift in wavelength with angle of incidence for a measured laser line filter at 532 nm.
It is possible to couple Alluxa’s ultra-narrow filters with additional filtering structures in order to create deep and wide blocking performance at state of the art levels (up to and beyond OD6) without sacrificing transmission performance.
Typical performance specifications are a 400 nm to 1100 nm range around the passband with OD4, OD5, or OD6 blocking performance. Blocking levels do not usually have an impact on transmission, but it can add to the cost of manufacture of the filter.
Optimizing Performance and Cost
The opportunity for optimal performance/cost balance in the finished filter is provided by the wide range of filter performance requirements that can be designed into Alluxa’s ultra-narrow bandpass filters. The most fertile areas to consider performance/cost tradeoff are: substrate material, blocking range and level, and spectral tolerances on filter attributes such as size and clear aperture, and center wavelength (CWL) and full width – half maximum (FWHM).
Although the blocking requirements do not significantly impact the transmission levels achieved in Alluxa’s filters, it does add cost and complexity to the design and coating process due to the need for additional layers. In many cases, the cost of the filter may be optimized by carefully addressing the exact blocking range needed.
Only the ASE spectrum of the laser may need to be blocked to high levels in laser cleanup situations. Alternatively, in a detection application, the blocking requirements will vary based on whether or not the system is using a silicon detector, PMT, or otherwise. Factoring in the responsivity of the detector can result in further reduction of the layer count in the filter and, therefore, the cost.
In terms of substrate material, Alluxa filters are generally coated on either UV grade fused silica or Borofloat 33 in different thicknesses and sizes. A sufficiently high quality, durable optical glass is provided by Borofloat 33 in many applications. In certain cases, the use of fused silica is necessary, especially in applications where there is extreme sensitivity to auto-fluorescence of the substrate material.
When considering the spectral profile of ultra-narrow filters, extra constraints on the filter shape such as FHWM and CWL tolerances can have a significant impact on cost. The easiest method for specification of the spectral profile is to call for the bandwidth to be less than a defined spacing and to call for an absolute transmission at a defined wavelength.
The filter in the above figure was specified in this way: > 92% Transmission at 532 nm and < 1 nm FWHM. Any extra tolerances to define the spectral shape should be taken into account in terms of their value to the application as the effect on cost can be large.
Flat top and square narrowband filters of less than 1 nm in bandwidth and transmission of > 90% even 95% are currently available at sizes of up to 250 mm in diameter. It is also possible to add out of band blocking levels of 3 OD to 6OD. These filters can be used for a variety of applications such as telecommunications, laser clean up, laser based remote sensing, LIDAR, other instrumentation applications.
This information has been sourced, reviewed and adapted from materials provided by Alluxa
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