Try to look at some of Newport products:

http://www.newport.com/Focusing-and-Collimating/141191/1033/content.aspx

I used to work on some optics experiments during my PhD. Since then, I have moved to MRI research. So, I do not remember all details.

For focusing the laser beam into small spot, I had some beam expander which I used to choose the central spot and then use lens to focus. Another approach was with fiber and some lens coupler. If you want, I would try to find schematic of the set up that I was using.

Additionally to the comment of Andrea V. Pipa, I would suggest to use not glass but quartz fiber due to its low absorption in ultraviolet, in particular at 337nm.

Having a 3-dB coupling loss is quite typical in laser-to-fiber coupling. Things to consider:

1) coupling to SM or MM? If it's MM then you can couple a "small" laser spot to a "large" fiber. Have to worry about satisfying NA of the fiber mostly. If it's SM then you have to satisfy mode matching condition as in the coupling integral.

2) a proper ARC on optical components used and a fiber coupler with a design wavelength close to 337 nm.

3) a proper fiber with minimal losses in the UV range

If you already have all three moments addressed, most likely an output shape of the laser beam is elliptical or so. Hence, mode matching is not there. To address it, you'd need to use a combination of free space lenses (aspherical, cylindrical etc, depending on a shape of the output laser beam) to build a customized fiber-optic coupler. Can be tricky...

The easiest way to improve the coupling with the coupler that you already have is to move along the 337-nm beam and locate the point where coupling maximizes given that those beams are usually divergent. In theory, there is an optimum point where the numerical apertures of the focusing lens and the optical fiber are identical. Do not expect more that 70% coupling efficiency as the 337-nm beam is most probably elliptical, and yes, you have to consider using a good UV-transmitting coupling lens and optical fiber; the later preferably as short as possible as even good UV fiber materials absorb. Good luck!

Efrain's answer just reminded me that when I used a Nitrogen laser last time (~15 years ago) it had a huge (!) output spot size that required using 10-15 cm diameter lenses. Do you also have such a spot? 

I would recommend to improve the quality of the laser beam on the nitrogen laser, due to the installation of an unstable resonator. M-resonator take a little while to lose energy. Then try using a short-focus lens to focus.

Thank you all for your valuable inputs. Currently, have been focusing using quartz lens and it works for me.