FB900-40 In Stock

The FB900-40 by ThorLabs is a soft-coated bandpass filter that is the perfect solution for transmitting a well-defined wavelength band of light while rejecting unwanted radiation. The FB900-40 features a design similar to a thin film Fabry-Perot Interferometer formed by vacuum deposition techniques, making it one of the simplest ways to achieve high transmission in the bandpass region.

The FB900-40 consists of two reflecting stacks separated by an even-order spacer layer. These reflecting stacks are constructed from alternating layers of high and low refractive index materials, with a reflectance in excess of 99.99%. By varying the thickness of the spacer layer and/or the number of reflecting layers, the central wavelength and bandwidth of the filter can be altered to suit the specific needs of the user. Please note that because of the Fabry-Perot design, the FB900-40 is designed to be used at an AOI of 0°. The transmission band may changed in terms of wavelength and can be reduced when used with another AOI. Moreover, the light that is blocked on both sides of the bandpass region has a very limited spectrum. To compensate for this, an additional blocking component is added to eliminate any unwanted out-of-band radiation. Depending on the requirements of the user, this blocking component may be an all dielectric or a metal-dielectric.

The FB900-40 offers a central wavelength of 700 nm to 1650 nm and is mounted in an anodized aluminum ring that is black and has an outer diameter of Ø1" and a maximum edge thickness of 6.3 mm. The aluminum ring makes handing easier and limits scattering, enhancing the blocking OD as a result. It is important to note that the FB900-40 is unthreaded, so retaining rings will be necessary to mount it into compatible internally-threaded lens tubes. Additionally, the filter has many layers of glass that are bound together with the mounting ring and epoxy, making it unadvisable to remove it from its mount. These glass layers are critical in the protection of the dielectric coating from the atmosphere, and the transmission efficiency of the filter will be reduced over time with continued exposure. Finally, it should be noted that the dielectric coating breaking down is caused by water vapor is gradual and results in the FB900-40 filter having a two year typical lifetime.