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Description  |
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The present invention relates particularly to helium-neon lasers of the
type emitting colors other than red.
Lasers have been known for many years and have many uses. The gain medium
may be gaseous, and one of the most commonly used gas lasers is the
helium-neon laser which commonly emits a red light.
In the simplest form of helium-neon laser the frequency of the emitted
light changes as the laser tube heats up. This is because the frequency
emitted is dependent on the length of the laser tube which expands as it
heats. In addition, most lasers emit light in more than one mode so that
the output is not a pure single frequency, but is composed of two or more
optical frequencies (cavity modes).
Many uses of a laser require the emitted light to be of a known and
constant single frequency. Consequently many helium-neon lasers use a
frequency stabilisation system. One such system controls the length of the
laser such that the frequencies of its two emitted modes are on either
side of the centre frequency of the neon transition. These two modes are
typically in orthogonal polarisation states. The intensities of these
modes are compared to provide a correction signal. If the modes change
frequency, the correction signal also changes and may be used in a
feedback circuit so as to prevent or reduce such frequency changes. The
correction signal is applied by means such as a heater or a piezo-electric
element, that can alter the length of the laser.
The heater may take many different forms, such as heater wires, thermofoils
or a thin film heater coated on the laser tube, the last being described
in an article by K. Seta and S. Iwasaki entitled `Frequency Stabilisation
of the He-Ne Laser using a thin film heater coated on the laser tube`,
Optics Communications, Volume 55, No. 5, October 1985, Elsevier Science
Publishers B V (Amsterdam, NL).
Another problem that occurs with lasers operating at high temperatures is
that the tube bends and becomes misaligned. A laser alignment system is
described in U.S. Pat. No. 4,010,363 which describes a rather complicated
system comprising a plurality of heaters along the tube. By use of control
circuits, selective portions of the tube are heated if the laser becomes
misaligned. The system is complex and requires positional misalignment
data collection and feedback signals.
Another form of stabilisation system, which is applicable particularly to a
laser that emits in only one mode, makes use of the Zeeman effect. A
magnetic field is applied so that the mode is split into two orthogonally
polarised components. In the stabilisation system, these components are
controlled to be on either side of the neon frequency. The correction
signal may be derived either from the relative intensities of the two
components, or from their frequency separation.
Such a stabilisation system is described in U.S. Pat. No. 3,649,930 where
the Zeeman effect is used in conjunction with the Doppler effect in a
stabilisation system for frequency stabilisation of a single-mode gas
laser.
It has recently been found possible to manufacture helium-neon lasers which
emit light of a color other than red. This can only be done by very
careful control of the helium to neon gas mix ratio and of the mirror
reflectivities. There are many potential uses for non-red helium-neon
lasers in applications where non-red light is required. For example a
non-red helium-neon laser can be used in conjunction with a red
helium-neon laser in multi-wavelength interferometry.
However in multi-wavelength interferometry, and in some other areas where
non-red helium-neon lasers might be used, it is important that the emitted
frequency be stable. Unfortunately it has been found that the system which
are so effective in stabilising red helium-neon lasers are not directly
applicable to non-red lasers. Reasons include laser emission in more than
two cavity modes (frequencies), the tendency of such modes to change their
polarisation states when the laser length changes very slightly, and the
extreme sensitivity of the laser to misalignment by bending as a result of
thermal disturbances.
There is therefore a need for an improved frequency stabilisation system
suitable for non-red helium-neon lasers.
According to one aspect of the present invention a helium-neon laser of the
type emitting radiation of the ranges of ultra-violet, infra-red and
visible, other than red, light, the laser tube emitting the radiation in
at least two modes, characterised in having a stabilising system
including, in combination;
a heating system adapted to supply heat substantially uniformly along the
length of the laser tube in order to bend the laser tube towards an
optimum alignment;
a magnetic field so adjusted as to reduce instability of mode polarisations
and to optimise relative intensities of the modes; means of frequency
modulation by cyclically varying the tube length; and means providing
active frequency stabilisation by means of control of the tube length, with
a stabilisation signal derived from the steady or varying intensity or
intensities of one or both of two orthogonally polarised optical outputs.
The alignment heating system may embody a strip heating element extending
along the length of the laser tube, or may utilise a metal strip or bar
that is separately heated.
The tube length may be cyclically varied by piezo-electric means, by
magnetic means, or by thermal means, the thermal means preferably being
effected by superimposing a varying electric current on an electric supply
to the heating system.
The frequency stabilisation signal may be derived either from a balance
between the intensities of the orthogonal polarisations, or from a
variation of intensity resulting from an imposed modulation of the laser
length, or from a sequential or simultaneous application of both these
techniques.
According to another aspect of the invention there is provided a method of
frequency stabilisation of a helium-neon laser of the type emitting
radiation in the ranges of ultra-violet, infra-red and visible, other than
red, light, the laser tube emitting the radiation in at least two modes,
characterised in that it comprises the steps of:
adapting a heating system to supply heat substantially uniformly along the
length of the laser tube in order to bend the laser tube towards optimum
alignment;
applying a magnetic field and adjusting it so as to reduce instability of
polarisations and to optimise relative intensities of the modes;
cyclically varying the tube length for frequency modulation; stabilising
the output frequency of the laser by means of control of the tube length
with a stabilisation signal derived from the steady or varying intensity
of one or both of two orthogonally polarised optical outputs.
One embodiment of the invention will now be described, by way of example
only, with reference to the accompanying diagramatic drawings, of which:
FIG. 1 shows a laser according to the invention, and FIG. 2 shows the
emission pattern of the laser illustrated in FIG. 1.
A laser (FIG. 1) has a metal or glass outer tube 10 and an inner capillary
11 at one end 12 of which is an anode 13. An electrical discharge takes
place between the anode 13 and the cathode, which may be the metal tube 10
or a separate metal tube within the tube 10. Partially transmitting
mirrors 14 and 15 are situated at the ends of the capillary. Light
emerging from mirror 15 passes through a polarisation-rotating element and
beam-splitter 18 such that only one plane of polarisation is transmitted,
and some of this transmitted light is partially reflected to a first
photodetector 50. Light in the orthogonal polarisation is reflected to a
second photo-detector 51. Electrical signals from both detectors pass
separately to a control unit 23.
Permanent magnets 24 and (dotted line) 25 are situated adjacent to the tube
10.
An alignment heater 19 extends over a substantial part of the length of the
tube 10. Tube 10 is surrounded by a coiled heating strip 20 or foil
heater, which may conveniently be in two parts so that one part may be
used for cyclic heating (modulation) and the other part for length
control. Power for the heaters comes from a unit 21, which has inputs from
the control unit 23 and an oscillator 22.
The tube contains a helium-neon gas mixture and operates in known fashion
as a laser when a discharge takes place through the capillary 11 by means
of the anode and cathode electrodes. The emitted light consists typically
of three substantially single frequencies (modes) f1, f2, f3, (31, 32, 33)
within a broad envelope 30 of the neon emission spectrum. These
frequencies are inversely proportional to the length between the mirrors
14 and 15 of the laser. As the tube heats up, the frequencies decrease. At
any time each one of the three frequency outputs will be in one of two
orthogonal polarisation states. When the laser length changes not only do
the frequencies change, but so also do the polarisation states.
The laser is required to provide a stable output, so that one of the
frequencies f1, f2, f3 is in a reproducible polarisation state, and the
other two frequencies are in the orthogonal polarisation state. Thus only
one frequency is transmitted by the polarising optics 18. Furthermore, the
transmitted output is stabilised in frequency by control of the laser
length in a particular manner detailed in this invention.
The permanent magnets 24, 25 are empirically positioned adjacent to the
tube 10 such that the laser outputs change polarisation much less
frequently than would otherwise be the case, and such that the tuning
envelope 30 has steeper flanks than otherwise. Thus the intensities of
outputs 31, 33 change more steeply of their frequencies are made to change
(e.g. by altering the tube temperature).
The tube 10 is mounted with the capillary 11 approximately horizontal and,
while operating as a laser, its position is adjusted by rotation about
this axis in order to maximise the output power. The power changes because
the temperature of the tube is greatest at the top and this affects the
optical alignment of the mirrors 14, 15 by differential thermal expansion
of the tube 10. The alignment is then optimised by mounting the alignment
heater 19 either along the top or bottom of the tube 10, depending on
whether more or less differential heating is required to improve the
alignment. Therefore, the tube is asymetrically heated (with respect to
its circumference) by heater 19, thus bending the tube due to differential
expansion.
The polarisation - rotating component incorporated in unit 18 is adjusted
so that the emitted polarisations are horizontal and vertical planes, and
part of the light of each polarisation is directed onto the separate
respective photodetectors 50, 51. Thus, for example, if output 31 is in
one state of polarisation and outputs 32 and 33 are in the other state
(see FIG. 2), then a small increase in frequency would cause output 31 to
increase in power and the sum of outputs 32 and 33 to decrease. This
differential power change is the basis of one type of control signal from
unit 23 that operates unit 21.
A second control signal results from the cyclic heating of part or all of
the length of the laser tube 10 by adjusting the heater 20 by means of the
oscillator 22 and power unit 21. The cyclic heating gives rise to a
corresponding cyclic length change and cyclic frequency change of the
outputs. These cyclic frequency changes give rise to corresponding
intensity changes at the photodetectors. The signal from either of the
photodetectors 50, 51 may be utilised, or a differential signal from both.
The resulting electrical signal can be selectively amplified and
synchronously detected with a phase sensitive rectification system in the
control unit 23, by means of a reference signal from the oscillator 22.
The output from the phase sensitive rectifier changes if the frequencies
of FIG. 2 change, corresponding to the gradients of the tuning envelope
30. The phase sensitive rectifier output may be compared with a fixed
reference voltage or with one derived from the direct output of the
photodetector, so that any frequency changes give rise to a correction
signal from unit 23.
In some applications of the invention the control signal based on the
differential power change will be convenient. This will be the case
particularly when cyclic changes of the optical frequency are undesirable.
In other applications, the control signal based on cyclic length changes
will be more satisfactory. In many cases, however, it will be advantageous
to use a combination of both control signals. In one example of a
combination method, the laser may be initially stabilised by the
differential power change signal so as to cause the output frequency to
approximate to a suitable value such that the cyclic type of control
signal can subsequently be used. This type of dual control serves to
reduce the possibility of the laser being stabilised at an undesired
frequency at which one or other of the control signals may have an
ambiguous value.
In another combination method, either one of the differential or cyclic
control signals may be used for stabilisation, with the other control
signal being used to discriminate against stabilisation at undesired
frequencies. Another example of a combination method is to stabilise the
laser primarily to the differential control signal, but to modify the
value of the differential power balance by reference to the cyclic control
signal. It would be particularly advantageous in this case to use the
integral of the cyclic control signal to modify the differential power
balance signal. Such a technique improves the long term stability of the
system.
It will be realised that there are many variations, within the scope of the
invention, of the stabilisation system described above. For example, foil
heaters, coils or strip heaters may be used for the alignment and length
adjusting heaters. Also, for convenience, laser length variation by means
of heat supply variation has been described. In many cases it will be more
convenient to use other means of length variation, or a combination of
several different means. In particular piezo-electric elements, or
magnetostrictive action on the mountings of one or both of the mirrors 14,
15 are especially convenient.
It will also be realised that the term light used in the specification is
intended to cover radiation in the ultra-violet, infra-red and visible
regions of the electromagnetic spectrum.
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