Coherent light generators – Particular beam control device – Optical output stabilization
Patent
1987-09-11
1989-03-28
Scott, Jr., Leon
Coherent light generators
Particular beam control device
Optical output stabilization
372 28, H01S 313
Patent
active
048171000
DESCRIPTION:
BRIEF SUMMARY
The invention relates to methods and apparatus for monitoring and controlling the frequency stability of a source of coherent radiation.
In coherent optical transmission, metrology, spectral analysis and holography there is often a need for lasers with a very stable reference frequency. The external cavity semiconductor laser offers very good short term stability but poor long term stability. By contrast a gas laser, which operates at a frequency determined by an atomic transition, has the potential for excellent long term stability as well as having very good short term stability, but its medium term stability over a period of a few minutes is often poor. This is due to temperature and mechanical variations which change the cavity length, which in turns shifts the mode frequency in the laser gain profile. In practice the frequency of an unstabilised gas laser, such as a HeNe laser, tends to cycle over a range determined by its gain/bandwidth characteristic and its cavity mode spacing. Choice of low expansion materials such as Invar for the laser cavity and careful temperature control will tend to increase the period of the cycles, but there will always be some uncertainty in the absolute frequency of the emission line within the gain profile. For a HeNe laser operating at 1.523 .mu.m this will be between about 300 MHz and 1 GHz (see paper by Eerkens J. W. and Lee W., "New HeNe lasers with 543 nm and 1523 nm outputs" presented at the conference on lasers and electro-optics (CLEO 85), Baltimore, Md., U.S.A., 21.mu.24 May 1985).
To achieve greater frequency stability it is possible to use automatic control techniques that rely on a particular spectral feature of the laser. One such feature which is commonly used is the Lamb Dip (Shimoda K, "Stabilisation of the HeNe maser on the atomic line centre", J.Opt.Soc.Am 1964, 54, p. 560). However such features are not always pronounced, particularly if other spectral features of the laser have been optimised, which may take stabilisation using this technique difficult to achieve or unreliable. Alternative laser stabilisation techniques have been developed which use absorption cells. In these, complex features of the absorption spectra of atoms or molecules are identified and interpreted to given an accurate measurement of optical frequency, but for unambiguous operation the control system needs to be complicated.
In accordance with one aspect of the present invention, a method of monitoring the frequency stability of a test source of coherent radiation relatively to a reference source of coherent radiation comprises repeatedly sweeping the frequency of a reference radiation beam generated by the reference source through a range of operating frequencies; causing the reference beam to interfere with the beam from the test source to generate a resultant beam with a beat frequency which varies between upper and lower values; and monitoring the variation of the average of the upper and lower beat frequencies with time.
In accordance with a second aspect of the present invention, apparatus for monitoring the frequency stability of a test source of coherent radiation relatively to a reference source of coherent radiation comprises reference source control means for repeatedly sweeping the operating frequency of a reference radiation beam generated by the reference source through a range of frequencies; interference means for causing the reference beam to interfere with the beam from the test source to generate a resultant beam whose frequency varies between upper and lower values; and monitoring means for determining the upper and lower beat frequencies and for monitoring the variation of the average of the upper and lower beat frequencies with time.
The invention avoids the need to identify complex features of either the output spectrum of a laser or of an absorption cell by using the fact that the gain profile width of certain sources, such as a HeNe laser, is very stable and that the cavity mode spacing is stable to first order and any second order effects on the mode s
REFERENCES:
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patent: 4468773 (1984-08-01), Seaton
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Cameron Keith H.
Smith David W.
British Telecommunications PLC
Jr. Leon Scott
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