Direct Frequency Comb Spectroscopy with an Immersion Grating

Summary form only given. Direct frequency comb spectroscopy (DFCS) in the mid-infrared region, especially in the long mid-infrared region (>5 μm), is a powerful technique for atmospheric remote sensing [1] and fundamental spectroscopy such as studies of C
₆₀
[2]. DFCS has been demonstrated in Fourier transform spectroscopy, dual-comb spectroscopy, and techniques involving a dispersive spectrometer. Although a dispersive spectrometer with a virtually-imaged phase array (VIPA) can probe transient molecules by recording spectra within 10 μs [3,4], it has the disadvantages of low throughput and limited-bandwidth, which present challenges for sensitive, broadband detection at high time resolution. In this work, we overcame the challenges of a VIPA spectrometer by developing a direct frequency comb spectrometer in the 8.5 gm region using an immersion grating. A preliminary demonstration was reported at 4 gm in this conference two years ago [5]. Figure 1(a) shows an overview of the spectrometer. The output light from 8-10 gm OPO-based frequency comb [6] enters an immersion grating at the blaze angle relative to the diffraction surface. An immersion grating is basically an echelle grating, but the diffraction surface is immersed in a highly refractive material, which is Ge in our case. The two main advantages of the immersion grating are that it has high resolving power owing to the high blaze angle of 75° and high refractive index of 4, and a high throughput of 50%. Since the diffraction order is 432 and the beam diameter is 34 mm, the higher orders of the diffracted light are spatially overlapped. To observe the full bandwidth of the comb spectrum, a cross -disperser consisting of a regular grating separates the overlapping orders along the orthogonal axis, and the resulting 2D image of the comb spectrum is imaged onto an infrared camera. Figure 1(b) shows a typical 2D image with N20 absorption. Since the resolution of the immersion grating is not sufficient to resolve the comb mode spacing (110 MHz), the comb spectrum is observed as a series of vertical fringes. From molecular absorption features, the immersion grating's free spectral range was measured to be 20 nm. The resolution of the spectrometer was determined from measurement of the pressure -dependent linewidths of N20 absorption lines. We obtained the resolution of 460 MHz with the beam diameter of 34 mm, which is the highest resolution demonstrated in this wavelength using a dispersive spectrometer. We also succeeded in obtaining a comb mode -resolved 2D image using a cavity -filtering technique. This spectrometer provides the capability to perform precise spectroscopy of transient and large molecules in the long mid -infrared region.