Spectrometers > Applications > Dispersive Raman Spectrometer

Recently, P&P Optica has developed a dispersive Raman spectrometer with CCD based detector, capable of capturing from samples both the fluorescence signal and the Raman signal. These new high performance dispersive Raman spectrometers can simultaneously register to over 1000 resolved spectral points in the spectral band of 380 to 1050nm. In the spectral band from 900 to 1700nm, the spectrometers can resolve 500 spectral points. Depending on the application, the systems can be built to cover from 0.2 to 0.95 of an octave in the above spectral ranges.

Figure 1. P&P Optica's high performance, dispersive Raman spectrometer used in these experiments. The spectrometer is attached to a Hamamatsu camera. As can be seen, the spectrometer is less than 12" without the collecting objective and camera. Modular design allows easy interchange of input methods such as c-mount lenses, f-mount lenses, fibers and others. The output of the spectrometer may also be customized and fits a wide range of cameras, including c-mount cameras.

In recent experiments, a pharmaceutical of interest was identified in a sample and a non-scanning broad-band imaging fluorescence spectrometer was used as a Raman spectrometer. A 20 mW 488nm Ar laser was used for the excitation wavelength but due to coupling losses, the light reaching the sample was 15 mW. The scattered and fluorescent light from the sample were collected using a standard photographic camera objective coupled to the c-mount equipped, 3mm long slit entry port of the high performance Raman spectrometer built by P&P Optica. The spectrometer was attached to a Hamamatsu CCD camera (1024 by 128 pixels, each 24 microns) with a back illuminated array, cooled to about -10°C.

The equivalent numerical aperture (N.A.) of the tested system is close to about 0.002 which means that its light collecting capability is about 250 times lower than a microscope having an objective with 0.4 N.A. Even with such unfavorable light collection conditions, the spectrometer was able to register a complete fluorescence spectrum (Fig. 2) with clearly visible Raman components (Fig. 3). This is due to the exceptional dynamic range and low scatter of the high performance spectrometer. As can be seen from the results with only 3mm slit and low end spectroscopic camera, the P&P Optica dispersive Raman spectrometer performs exceptionally well. The Raman peaks are well defined, with very minimal noise present in the spectrum. Furthermore, the performance of the spectrometer can be improved by using a larger array (12mm along the slit direction and up to 24.5 mm in direction of spectral dispersion) with appropriate detector selection up to 1000 spectral points can be resolved.

Figure 2. Acquisition of Raman Spectra. As can be seen, the exposure of the sample had both fluorescence and Raman signal superimposed. It was necessary to expose the sample holder only to subtract the fluorescence signal to get the final Raman spectra. It should be noted that in this case, the sample in the sample holder absorbed some of the fluorescence from the sample holder, giving a "negative" peak. This was corrected for the final Raman spectrum..

Figure 3. The above figure shows the Raman spectra obtained upon subtraction of fluorescence signal from the obtained data. The top plot shows the entire spectrum, and the plot below shows a detailed view of the Raman signal. As can be observed, there is a good definition of peaks and minimal noise. It should be noted that signal was collected in less than an optimal way, yet good spectral definition is visible.

Features of P&P Optica's dispersive Raman spectrometers

Although the data presented above shows a system using the spectrometer attached to 3 x 25mm CCD detector, P&P Optica spectrometers have been designed to provide aberration free operation for 12 x 25 mm detectors. The input slit can therefore be up to 12 mm long and as narrow as 10 microns. The Raman spectrometers are diffraction limited to pixel size under 10 microns, so that even 10 micron pixels may be used to improve spectral resolution. This provides unprecedented spectral resolution in large input slit, non-scanning spectrometers.

(For typical issues regarding scanning spectrometers please refer to the application note about benefits of non-scanning systems)

Features of P&P Optica’s Raman Spectrometers

  1. Simultaneous registration of spectrum up to one octave wide, particularly useful for spectral analysis of non-stable signals.
  2. Low noise, highly efficient Volume Phase Holographic (VPH) diffraction grating.
  3. Patented, transmission based optical design with low noise and low aberrations provides a high quality spectral image for each point along the entrance slit which can be as long as 12 mm.
  4. Binning across the entire slit length does not affect the spectral resolution of the Raman spectrometer.
  5. Measurement of up to 1000 separate spectral points simultaneously is possible in ranges as narrow as 0.15 of an octave and as wide as 0.95 of the octave within spectral band 380nm to 2150nm. This is limited only by the availability of photodetector arrays.
  6. Modular design allows for modification of the spectrometer to meet even the most demanding requirements of customers in respect to resolution and spectral band.

Summary

The P&P Optica dispersive Raman spectrometers provide several benefits over typical systems available on the market today. Although typical non-scanning systems provide lower spectral resolution than scanning spectrometers, the introduction of transmission based design which significantly improves the dynamic range and reduces aberrations allows P&P Optica to provide its users with exceptionally high spectral resolution in a dispersive system. The modular design of the Raman spectrometer also provides the users with flexibility regarding their experimental setup.

Available Dispersive Raman Systems

Excitation wavelength [nm] Raman shift range [cm-1] Resolution [cm-1]
785
1620
1.7
830
1520
1.6
1064
1200
2.5
532
3640
3.7
633
3140
3.2
785
2530
2.6
830
2340
2.4
1064
1880
3.9
405
6840
6.9
441
6280
6.4
488
5790
5.9
514
5370
5.5
532
5280
5.4
633
4430
4.5
785
3210
3.3
830
2520
2.6
1064
2640
5.4
405
9070
9.2
441
8390
8.5
488
7670
7.8
514
7260
7.4
532
7030
7.1
633
5890
6.0
1064
3510
7.1
405
11870
12.0
441
10910
11.0
488
9970
10.1
514
9450
9.6
532
9090
9.2
633
6270
6.4

 

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