Coherent light generators – Particular beam control device – Mode discrimination
Patent
1990-07-16
1991-12-31
Sikes, William L.
Coherent light generators
Particular beam control device
Mode discrimination
372 54, 372 70, 372 93, 359333, H01S 3098
Patent
active
050777467
DESCRIPTION:
BRIEF SUMMARY
This invention relates to lasers and, in particular, to dye lasers. Dye lasers are widely used as sources of frequency tunable, nearly monochromatic light. The spectral width of such lasers can be made more nearly monochromatic by arranging the laser to operate on a single longitudinal mode. The conventional approach to achieving this narrow bandwidth, single mode operation, is to place in the laser resonator some kind of frequency selective element such as a prism, diffraction grating or interference filter. These elements usually introduce large losses and so reduce the Q of the cavity as well as necessitating a long enough cavity length to accommodate them. The chief limitation of these devices lies in the difficulty of tuning the frequency once a single mode has been established. This requires simultaneous adjustment of the frequency selective element and the cavity length, L, since the frequency, .nu., must satisfy the mode condition: tolerances on the accuracy and stability of mechanical components, their dimensions, positioning and movement must be controlled to within one half a wavelength of light. Failure to remain within these tolerances leads to frequency instability and a very restricted tuning range.
We have adopted a completely different strategy to produce tunable single mode pulsed dye laser radiation. In our device, we use a short, high-Q cavity which emits a comb of longitudinal modes spanning the gain bandwidth of the dye. The single mode selection is done outside the laser cavity.
According to the present invention there is provided apparatus for generating a beam of substantially monochromatic coherent radiation comprising a resonator device for generating a plurality of modes of said radiation together with means external to said resonator device for selectively amplifying a predetermined one of said modes.
The invention will now be particularly described with reference to the accompanying drawings in which:
FIG. 1a to 1c is a series of plots of radiation intensity v frequency used for explanation of the operation of embodiments of the invention;
FIG. 2 is a diagrammatic representation of the arrangement of a short cavity length dye laser;
FIG. 3a and 3b illustrate pumping arrangement for the laser of FIG. 2;
FIG. 4a is an elevation of a narrow bandwidth amplifier for use with a short cavity length laser and FIG. 4b is a plan view of the same;
FIG. 5 is and explanatory plot of intensity v frequency; and
FIG. 6 is a diagrammatic representation of the layout of apparatus in accordance with one aspect of the invention.
In a specific embodiment of the invention, a tunable single-mode pulsed dye laser uses a novel strategy to produce monochromatic radiation. In this device a short high-Q cavity emits a comb of longitudinal modes spanning the gain bandwidth of the dye. Single mode selection is performed outside the cavity by means of a narrow bandwidth amplifier system. The spacing of the modes from the oscillator is to satisfy the condition: .DELTA..nu..sub.Amp <.DELTA..nu..sub.m. Thus only one mode is effectively amplified and appears in the output. Tuning of the device is achieved by adjusting the length L of the short cavity laser (SCL). Since there are no intra-cavity elements to be adjusted simultaneously, the design and construction of the device is simplified. It is sufficient merely to adjust the centre frequency of the narrow band amplifier (NBA) such that the amplified band of width .DELTA..nu..sub.Amp includes the mode being selected. Since the spectral width .delta..nu. of any mode from the SCL is very narrow, .delta..nu.<<.DELTA..nu..sub.m, it is relatively simple to ensure that this mode stays within the gain bandwidth of the NBA.
The tuning range of the device is determined by the change .DELTA.L in the SCL length and is given by:
The principle of using a very short cavity to achieve single mode operation is well known in the case where the mode spacing .DELTA..nu..sub.m can be made to exceed the bandwidth of the gain medium e.g. in diode lasers or solid state lase
REFERENCES:
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Applied Optics, vol. 25, No. 12, Jun. 15, 1986, Y. J. Xie et al.: "Near Transform-Limited Picosecond Dye Laser Pulses by Cascaded Transient Oscillation", pp. 1877-1878 see p. 1877, column 2, lines 32-35, 44-46; FIGS. 1-4.
IEEE Journal of Quantum Electronics, vol. QE-22, No. 12, Dec. 1986, IEEE, (New York, USO, K. Bohnert et al.: "Tunable Near-Infrared Picosecond Pulses from a Short-Cavity Dye Laser", pp. 2195-2199 see abstract; FIG. 1.
Hansen Galen J.
National Research Development Corporation
Sikes William L.
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