Compact and robust spectrograph

An apparatus for analyzing a sample using Raman spectroscopy comprising:

• a Raman sample cell;
• a laser source for producing light having an excitation wavelength in the range from 400 nm to 870 nm and a bandwidth less than 30 GHz, wherein said laser source is coupled to the Raman sample cell to direct light into the Raman sample cell;
• an optical connector for connecting one or more collection optical fibers to the sample cell for receiving scattered light from the sample cell as an optical signal;
• a Raman spectrograph comprising:
• one or more collection optical fibers, having an optical slit affixed at an end thereof, wherein the slit has a width in the range from 20 to 150 .mu.m;
• a collimating lens configured to receive the optical signal that passes through the slit, wherein the slit is disposed between the collection optical fibers and the collimating lens, wherein the collimating lens has an f-number in the range from f/1 to f/2 and a focal length in the range from 25 mm to 75 mm, wherein the collimating lens consists of multiple spherical elements;
• a reflective diffraction grating configured to receive the optical signal from the collimating lens and to disperse and reflect the signal to a focusing lens, wherein the reflective diffraction grating has between 600 and 1800 grooves/mm and a diffraction efficiency greater than 70%;
• said focusing lens configured to receive and focus the optical signal from the reflective diffraction grating, wherein the focusing lens has an f-number in the range from f/1 to f/2 and a focal length in the range from 25 mm to 75 mm, wherein the focusing lens consists of multiple spherical elements;
• and
• a light sensor for converting the optical signal into a corresponding electronic signal;
• and
• a computer for converting the electronic signal into a representation of the analysis of the sample.