Double subtractive spectrometer as a tunable high-resolution broad-bandpass optical filter

In the optical part of the electromagnetic spectrum, technologies such as dielectric and holographic filters provide a broad-bandpass function with poor resolution between fully blocking and fully transmitting and with almost no tunability. Standard double subtractive spectrometers provide a high-resolution filter function that is tunable over a wide range, but because of the placing of the dispersive element relative to the collimating optics, these instruments are intrinsically narrow-bandpass. A double subtractive spectrometer in which the resolution and tunability are not compromised, yet the bandpass may be as broad as may be desired, can be achieved by placing the dispersive element at twice the focal length from the collimating optics. Such an instrument is described and demonstrated in low-wave-number Raman spectroscopy. With a 0.2 m focal length, it achieves better than 1 cm(-1) resolution and can work to 5 cm(-1) of the laser line. Throughput is up to 35%, and a bandpass of 1000 cm(-1) is demonstrated. Combined with a final stage spectrograph, the laser rejection is 10(-13) at 50 cm(-1). (C) 2002 American Institute of Physics.