WO1994021012A1

WO1994021012A1 - Acousto-optic device

Info

Publication number: WO1994021012A1
Application number: PCT/GB1994/000371
Authority: WO
Inventors: David Colin Hanna; William Andrew Clarkson
Original assignee: University Of Southampton
Priority date: 1993-03-01
Filing date: 1994-02-24
Publication date: 1994-09-15
Also published as: GB9304077D0

Abstract

An acousto-optic modulator in a ring laser consists of a transducer (10) bonded to a deflection medium (11). The deflection medium is a laser-acted medium which, in use, acts as the gain medium of the laser. Acoustic waves are induced in the deflection medium (11) to effect deflection of light transmitted therethrough. The modulator can, thus, be formed in unitary fashion with the gain medium of the ring laser producing a robust and stable device.

Description

Acousto-Optic Device

This invention relates to a device for enforcing

> unidirectional operation of a ring laser.

Enforcing unidirectional operation of a ring solid-state laser and thereby avoiding spatial hole burning as described in "single-frequency travelling-wave NdrYAG laser" (Clobes, A.R. and Brienza, M.J., Appl. Phys. Lett., 21,265 [1972]) can be an efficient way to achieve narrow-linewidth, single-frequency output radiation. Such solid-state ring laser devices have potential applications in a number of important areas including remote sensing, coherent laser radar, metrology, coherent communications and high-resolution spectroscopy.

A variety of techniques exist for achieving unidirectional operation of a ring laser, but many of them suffer certain drawbacks which either limit their applicability or adversely affect the laser performance. The most common technique makes use of an intracavity Faraday isolator which, in its most basic form, consists of three separate components; a Faraday rotator (to provide a non-reciprocal polarization rotation), a reciprocal rotator, and a polariser. The effect of the reciprocal and non- reciprocal rotators is to produce different eigenpolarisations for the two counter-propagation directions which subsequently experience different amounts of attenuation at the polariser. The result is unidirectional lasing in the lower loss direction. Unfortunately the requirement for three extra components inevitably leads to an increase in cavity loss which can in turn lead to a degradation in the laser efficiency (especially for low-gain lasers), and furthermore, makes miniaturisation of the laser resonator extremely difficult. One solution to these problems is to use the laser medium itself as the Faraday rotator, together with a nonplanar ring geometry to provide the reciprocal polarisation rotation see "Monolithic, unidirectional single-mode Nd:YAG ring laser (Kane, T.J. and Byer, R.L., Opt. Lett., 10,65 [1985]). If this approach is adopted the number of cavity components, and therefore the cavity loss, can be considerably reduced. Indeed by arranging for oblique reflection from a dielectric mirror to provide the polarisation discrimination, the resonator can be made monolithic. Unfortunately, this approach is only applicable to those laser materials which have a large enough Verdet constant at the lasing wavelength and, since the technique relies on polarisation discrimination, can only be used reliably with laser materials which are not birefringent.

An alternative technique for enforcing unidirectional operation is via the acousto-optic effect. In this case only a single extra intracavity component is required, namely a travelling-wave acousto-optic (A-0) modulator. One of two approaches may now be employed since there are two quite distinct mechanisms by which acousto-optically induced unidirectional operation can be achieved. The first of these relies on an intrinsic property of all travelling-wave A-0 modulators as described in Clarkson, .A. , Neilson, A.B. and Hanna, D.C., ("Explanation of the mechanism for acousto-optically induced unidirectional operation of a ring laser," Opt. Lett., 17, 601 [1992]) which is that the Bragg condition cannot be exactly satisfied simultaneously for two counter-propagating laser beams. Thus when radio frequency (r.f.) power is supplied to the A-0 modulator and it is aligned so that one beam satisfies the Bragg condition then the counter-propagating beam cannot precisely satisfy the Bragg condition and will consequently experience a lower diffraction loss. Under these circumstances, and providing that the difference in the diffraction losses for counter- propagating beams is sufficiently large, then unidirectional lasing will occur preferentially in the direction of lower loss. In practice, a larger difference in the diffraction losses can often be achieved by tilting the A-0 modulator away from the Bragg angle as described in the Opt. Lett., reference mentioned above.

Alternatively, a second technique for A-0 induced unidirectional operation can be adopted, which involves feeding back the diffracted beams via a feedback resonator. This method is described in detail in Clarkson, W.A., Neilson, A.B. and Hanna, D.C., "Acousto-optically induced unidirectional operation of a ring laser: a feedback mechanism," Opt. Comm., 91, 365 [1992] and is known as the feedback technique. The basic principle relies on the fact that the counter-propagating diffracted beams in the feedback path have different frequencies, since one is upshifted by the acoustic frequency and the other downshifted. As a consequence, the round-trip phase shifts experienced by these beams along the feedback path are different, hence the effective diffraction losses experienced by counter-propagating beams in the main laser cavity are also different, and unidirectional lasing occurs preferentially in the lower loss direction. The choice of which of the two A-0 techniques is most suitable depends on the details of the particular laser, the resonator and modulator design, and on the desired mode of operation (i.e. continuous wave or Q-switched). In many situations either technique works perfectly well.

Acousto-optic techniques for enforcing unidirectional operation of ring lasers offer a number of benefits over the Faraday isolator approach. In particular, since they do not rely on polarisation discrimination they can readily be used with birefringent laser materials. The acousto-optic approach to unidirectional operation also offers the advantages of relatively low insertion loss (since only a single extra intracavity component is required) and unidirectional operation over a wide spectral range. Additionally, the acousto-optic modulator can also be operated as a Q-switch to obtain high peak power, pulsed, single frequency operation. Nevertheless the acousto-optic techniques for unidirectional operation that have been applied so far suffer from the disadvantage that an extra intracavity component (i.e. the acousto-optic device) is still required which inevitably leads to an increase in cavity loss. Even though this loss may be very small it can be very serious for low-gain lasers, severely degrading the lasing efficiency, and in some cases preventing lasing altogether. In addition, the requirement for an extra component limits the extent to which further miniaturisation can be achieved. In many applications the ability to design very compact laser systems is not only important from the point of view of minimising space and material requirements and therefore reducing the cost, but also for a number of technical reasons, in particular the much greater ease with which a single axial mode can be selected and tuned in frequency.

In accordance with the invention, there is provided an acousto-optic device for enforcing unidrectional operation in a ring laser, the device comprising a medium in which travelling acoustic waves ^"are induced, in use, in order to effect deflection of light transmitted therethrough, the device being characterised in that the said medium is a laser-active medium which, in use, acts as the gain medium of the laser.

In the present invention the acousto-optic unidirectional device is fabricated from the laser material itself. This is a novel and very useful extension of existing acousto-optic techniques, which avoids the problems outlined above, and is therefore expected to have far-reaching consequences for the improved design of miniature single-frequency ring lasers and therefore will have important industrial applications. Many solid-state laser materials are themselves not considered as suitable candidates for most acousto-optic applications since their acousto-optic figure of merit is generally small, and therefore they cannot provide sufficient diffraction loss at reasonable radio frequency powers. It is a general feature of the acousto- optic techniques for enforcing unidirectional lasing outlined above, that only a very low acousto-optic figure of merit is required, hence many solid-state laser materials are themselves suitable as the acousto-optic medium. This allows for the design of much simpler and more compact ring resonators, which employ fewer intracavity components and therefore, as a consequence of the reduced resonator loss, have improved efficiency. In addition, it is also possible to construct the ring resonator entirely from the laser material. Such monolithic resonators are very robust and can produce a very stable and reliable single frequency output.

A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which:-

Figure 1 shows a side view of a travelling-wave acousto-optic modulator fabricated from a solid-state laser material, and

Figures 2,3 and 4 are examples of ring lasers which incorporate an acousto-optic modulator, where it serves as both the laser gain medium and the device for enforcing unidirectional operation.

With reference to figure 1, the acousto-optic modulator is of the usual construction consisting of a transducer 10, bonded to the deflection (acousto-optic) medium 11, which in this case is also the solid-state laser gain medium (e.g. neodymium-doped phosphate glass). Acoustic waves are generated in the deflection medium by applying a radio-frequency (r.f.) drive signal to the transducer via electrodes located on its upper surface. When an incident laser beam 12 enters the modulator at or close to the Bragg angle

13, some of the light is diffracted to produce a diffracted beam

14. When used as a unidirectional device in a ring laser it is desirable (though not always essential) for acoustic waves to be absorbed after propagating through the deflection medium in order to prevent a standing-acoustic wave pattern being set up which would adversely affect the performance. For this reason, and in common with many other applications (e.g. Q-switching) , an absorbing medium 15 is bonded to the lower surface of the modulator. In addition, and as an extra precaution against the occurrence of standing-waves, it is also the usual practice to angle the lower surface 16 of the deflection medium 11. The cross-sectional shape of the acousto-optic modulator depends on the design of the ring resonator configuration to be used. This is illustrated in the examples shown in figures 2,3 and 4.

Figure 2 shows a ring laser with a triangular configuration defined by mirrors 17, 18 and 19, at least one of which must be curved for cavity stability. The acousto-optic modulator 20, in this case, has a rectangular cross-section with anti-reflecting dielectric coatings on its two end faces. An alternative ring resonator configuration is shown in figure 3, which consists of only two mirrors 21 and 22, and a rhomb-shaped acousto-optic modulator 23 such as is described in Clarkson, W.A., and Hanna, D.C., "Acousto-optically induced unidirectional single mode operation of a Q-switched miniature Nd:YAG ring laser," Opt. Comm, 81,375 [1991]). The design of this resonator is such that the laser beam strikes each of the four faces of the modulator at Brewster's angle in order to minimise the cavity loss. Both of these lasers can be pumped longitudinally by a second laser (e.g. a diode laser).

Unidirectional operation can be achieved via one of two techniques, which both rely on the travelling-wave nature of the acousto-optic device. The first technique, as mentioned above, makes use of an intrinsic property of all travelling-wave acousto-optic modulators, namely that the Bragg incident angle (that is, the angle that the incident laser beam makes with the acoustic wavefronts for the maximum diffracted power), is different for oppositely travelling beams. Thus, the Bragg condition cannot be satisfied simultaneously for both counter-propagating beams and as a consequence they generally experience different diffraction losses. It is this difference in the diffraction losses which can be used to enforce unidirectional operation. The procedure involves applying radio-frequency power to the acousto-optic modulator and tilting the modulator away slightly from the nominal Bragg angle so as to increase the loss difference. The magnitude of the loss difference depends on a number of factors including; the acousto- optic modulator design, its orientation, the acousto-optic properties of the deflection medium and the radio-frequency power and drive frequency. By the appropriate choice of these parameters unidirectional operation can usually be achieved. It is the normal procedure with this technique to add an aperture in the laser cavity to prevent multiple reflections, between the cavity mirrors, of the diffracted beams. This avoids the feeding back of the diffracted beams into the acousto-optic modulator which can give rise to changes in the value of the loss difference.

In the event that the loss difference required cannot be achieved by this technique, or if in the process of achieving it the diffraction loss required is relatively large so as to cause an unacceptable decrease in lasing efficiency, then an alternative method of enforcing unidirectional operation is the feedback technique outlined above. This makes use of the fact that the diffracted beams 14 and 24 of Figure 1, corresponding to counter-propagating laser beams 12 and 25 are respectively down-shifted and up-shifted in frequency by the acoustic frequency. The procedure involves feeding back the diffracted beams into the acousto-optic modulator so that they approximately re-trace their original paths. This can be done with additional mirrors or alternatively, if the appropriate laser resonator is used, by the laser mirrors themselves. Since their frequencies are different they generally experience different phase shifts after one round- trip of the feedback path and, as a consequence, also experience different effective diffraction losses. This can cause a sufficiently large loss difference for enforcement of unidirectional operation even with laser materials having a very low acousto-optic figure of merit.

It should be stressed that the two resonator configurations described here are only examples, and there are many other possible resonator configurations which could be used. A particularly attractive feature of this invention is that the resonator can be monolithic and hence fabricated entirely from the laser medium. A typical example of such a ring laser is illustrated in figure 4, where the laser gain medium 25 is also the acousto-optic modulator and the mirrors 26 and 27 are coated directly on to the laser medium. In the example shown the ring path is completed by a total internal reflection 28 at the boundary 29 between the laser medium and air. Monolithic ring resonators would offer the advantages of being extremely compact and robust, and would offer the additional advantage of very stable operation without necessitating the use of complex and expensive stabilisation electronics. Since the acousto-optic figure of merit needed for enforcing unidirectional lasing is extremely small it is anticipated that most, if not all, solid- state laser materials will have a large enough figure of merit. This will ultimately allow the construction of a variety of very stable, monolithic, single-frequency ring lasers with a diverse range of operating wavelengths.

Claims

1. An acousto-optic device for enforcing unidirectional operation in a ring laser, the device comprising a medium in which travelling acoustic waves are induced, in use, in order to effect deflection of light transmitted therethrough, the device being characterised in that the said medium is a laser-active medium which, in use, acts as the gain medium of the laser.

2. A device according to claim 1 operable to Q-switch the laser so that it provides a pulsed output.

3. An acousto-optic device for enforcing unidirectional operation in a ring laser, the device being substantially as hereinbefore described with reference to any of the drawings.

4. A ring laser including an acousto-optic device in accordance with any of claims 1 to 3.

5. A ring laser according to claim 4 in which the acousto-optic device is formed integrally with a laser resonator to provide a monolithic laser device.