LightMachinery recently shipped a new model from our line of etalon-based cross-dispersion spectrometers, the HN-9332-UHR-RS. This spectrometer delivers very high resolution and excellent wavelength accuracy over a wide wavelength range. The spectrometer has a small form factor and does not require any alignment adjustments or scanning to record more than 25,000 spectral elements in a single exposure.
The HN-9332-UHR-RS contains a proprietary etalon that has a finesse >100 across the entire 400 nm range of the spectrometer (485 to 885 nm). The high finesse enables the spectrometer to deliver a resolution of better than 10 GHz at all wavelengths. The resolving power, Δλ/λ, of the spectrometer varies from 60,000 near 500 nm to ~40,000 near 800 nm. Figure 1 shows the measured resolution in pm as a function of wavelength.
Figure 1 – Resolution as measured using four different laser light sources. Also shown is a line representing a constant resolution of 9.5 GHz.
With such a high resolution and such a large wavelength range, accurately calibrating this type of spectrometer can be a challenge. For the HN-9332 and HF-9332 models we make use of a second external etalon that generates hundreds of transmission peaks across the spectral range. The wavelengths of these peaks are known very accurately and are derived from the material properties and the coatings of the external etalon. All that remains is to use the known wavelength of a single spectral line from a reference lamp to complete the calibration. Figure 2 shows typical measurements of the wavelength accuracy of the spectrometer after the calibration process is completed. Note that the maximum error is only +10, -15 pm across the entire wavelength range.
Figure 2 – Measured wavelength accuracy of the HN-9332-UHR spectrometer after calibration. Each data point represents a separate reference line (lamp and laser emission lines, and solar Fraunhofer absorption lines). The wavelength error is the difference between the measured wavelength and the literature value for the source.
One of the major challenges faced by all cross-dispersion spectrometers is the issue of FSR cross-talk. This phenomenon occurs when an intense narrowband light source passes through the spectrometer and forms a series of spots when imaged onto the sensor. Some of the intensity at the edge of each spot may “bleed” into the positions of adjacent etalon orders, resulting in weak artefacts at wavelengths separated by one etalon FSR (Free Spectral Range) from the primary intense source. FSR cross-talk tends to be worse in the blue end of the spectrum, as the etalon orders are more closely spaced in that region of the spectrum. Figure 3 shows the measured cross-talk for the HN-9332-UHR spectrometer. Note that the maximum FSR cross-talk is only 0.2%, significantly less than the value of ~1% often quoted for competing spectrometers with similar cross-dispersion designs.
Figure 3 – Measure crosstalk (using four laser sources) for the HN-9332-UHR spectrometer.
All HN-9332-UHR spectrometers are fully aligned and calibrated at LightMachinery before shipment. No further alignment adjustment is required in the field, and the calibration accuracy shown in Figure 2 is maintained over periods of many months by means of an occasional one-minute software optimization process. These spectrometers are designed for use with bright light sources, and the spectrometer sensor can capture all the spectral information in a single exposure (no scanning of optical elements is required). 400 nm of wavelength range with an average resolution of ~15 pm implies that the spectrometer can capture >25,000 resolution elements in the time of a single exposure, which is often as short as tens of milliseconds. For comparison, a standard grating spectrometer captures 1000 or 2000 spectral elements in a single exposure, as limited by the single dispersion axis. In addition, a standard grating spectrometer that can deliver the resolution shown in Figure 1 typically has a maximum dimension of ~1 meter to accommodate the long-focal-length optics required for such high resolution. In contrast, as shown in Figure 4, the HN-9332-UHR optics can fit into a box with a maximum dimension of 20 cm.
Figure 4 - Photograph of the HN-9332-UHR-RS spectrometer. The dimensions of the enclosure are 20x20x8 cm3.
A spectrometer with a performance capability as unique as the HN-9332-UHR-RS will find applications in laser characterization, solar spectroscopy, and any applications that require fast spectral measurements with high resolution over a wide spectral range.