Portal

1. A portal or gateway configured to allow people or items to pass through, wherein the portal or gateway comprises:

a vertical support;

a chirped laser;

an open optical sample cell; and

a detector for detecting light from the chirped laser that has passed through the open optical sample cell, wherein the open optical sample cell is defined by a mirror at each end, the mirrors being included in the vertical support, so that the open optical sample cell extends along the vertical support.

2. A portal or gateway as claimed in claim 1 wherein the open optical sample cell extends along substantially the full length of the vertical support.

3. A portal or gateway as claimed in claim 1 wherein the open optical sample cell is longer than approximately 1 meter.

4. A portal or gateway as claimed in claim 1 , wherein the open optical sample cell is non-resonant.

5. A portal or gateway as claimed in claim 1 wherein the open optical sample cell is arranged so that light makes multiple passes through the open optical sample cell.

6. A portal or gateway as claimed in claim 1 further comprising means for directing gas in the portal towards the open optical sample cell.

7. A system comprising a plurality of portals or gateways, wherein at least one of the plurality of portals or gateways is defined as in claim 1 .

8. A portal or gateway as claimed in claim 1 wherein the chirped laser comprises a semiconductor laser.

9. A portal or gateway as claimed in claim 5 wherein the open optical sample cell is arranged so that light makes one hundred or more passes through the open optical sample cell.

10. A portal or gateway as claimed in claim 6 wherein the means for directing gas comprise one or more fans.

11. A system as claimed in claim 7 wherein the plurality of portals or gateways share a single detector.

12. A system as claimed in claim 11 wherein light from the plurality of portals or gateways is passed to the single detector using fiber optic cable.

13. A portal or gateway as claimed in claim 8 wherein the light is generated by applying one or more substantially step function electrical pulses to the semiconductor laser to cause the semiconductor laser to output one or more light pulses for injecting into the open optical sample cell, wherein each of the one or more light pulses comprises a continuous wavelength chirp.

14. A portal or gateway as claimed in claim 13 wherein each of the one or more light pulses has a duration that is greater than approximately 150 ns.

15. A portal or gateway as claimed in claim 13 wherein each of the one or more light pulses has a duration that is in the range of approximately 150 to 300 ns.

16. A portal or gateway as claimed in claim 13 wherein a chirp rate of the one or more light pulses is selected so that there is a time delay between spots on reflecting elements of the open optical sample cell sufficient to substantially prevent light interference from occurring, wherein the spots define locations at which at least one of the one or more light pulses is reflected from walls of the open optical sample cell.

17. A portal or gateway as claimed in claim 16 wherein each of the one or more light pulses has a duration that is greater than approximately 150 ns.

18. A portal or gateway as claimed in claim 16 wherein each of the one or more light pulses has a duration that is in the range of approximately 150 to 300 ns.