Mirrors of polarization sensitive reflectivity

Abstract: A mirror for a ring laser gyroscope comprising a multi-layer dielectric stack (9) and a layer having an optical thickness of a half wavelength (10) so as to enhance absorption of s-polarised radiation relative to that of p-polarised radiation.

Description: This invention relates to mirrors and is more particularly concerned with the mirrors used in ring laser gyroscopes. As background, there are two principal modes of polarisation obtained when plane polarised light is reflected from a surface. These modes are defined by the relationship between the electric vector (E vector) of the incident light and the plane of incidence. Using the convention given in "Optical Properties of Thin Solid Films" by O.S. Heavens published by Butterworth Scientific Publications (1955), p-polarised light is defined as having the E vector in the plane of incidence and s-polarised light as having the E vector normal to the plane of incidence. In general, if the incident radiation is p- or s-polarised, the plane of polarisation of the reflected light is unchanged. However, if the surface is made from a magneto-optically active material this does not hold. Furthermore, if the incident polarised light is polarised in any other plane than the plane of reflection, the plane of polarisation of the reflected radiation will, in general, be rotated by the reflection. Ring laser gyroscopes exhibit the phenomenon of "lock-in", where at low rotation rates, the two counter-rotating beams are locked to the same frequency and no frequency difference is observed until a certain higher rotation rate is attained. "Lock-in" is due to an interaction between the two lasing beams and even above the certain higher rotation rate, the effects of the interaction between the two beams is shown by the way they make the scale factor non-linear. The effect of "lock-in" can be overcome practically by providing a bias mechanism. The most commonly used method of providing this bias mechanism at present is the application of a dither frequency to the ring laser gyroscope. However, of several non-mechanical methods which have been suggested, the most attractive alternative to dither is the so-called "magnetic mirror". This was proposed in US Patent 3,851,973 and discloses the use of the magneto-optic properties of pure iron in the infrared region of the electromagnetic spectrum. Alternative "magnetic mirrors" using ferromagnetic garnet materials are disclosed in UK Patent 2006456 and US Patent 3,927,946. Magnetic mirrors apply a differential phase shift to the counter-rotating beams due to the transverse Kerr effect but only when the beams are p-polarised. Therefore, to utilise this phenomenon it is essential that the ring laser gyroscope should lase in a p-polarised mode. This presents no difficulty in a modular lasing cavity where the mode of polarisation is defined by the orientation of the Brewster angle windows on the discharge tube. However, an integral lasing cavity preferentially lases in the s-polarised mode because the mirrors currently employed in ring laser gyroscopes exhibit a greater reflection coefficient for s-polarised light than for p-polarised light. For two dielectric materials forming an interface, the reflection coefficient, R, at the interface is greater for s-polarisation (R s ) than for p-polarisation (R p ) as illustrated in Figure A. This difference is enhanced by a multilayer dielectric mirror of the type used in ring laser gyroscopes which is made of alternating layers of two dielectric materials having different refractive indices so that the ratio R p /R s is always less than 1. At normal incidence, however, there is no distinction between the two polarisations. When absorption loss is taken into account the ratio of Rp/ Rs remains less than 1 but the reflectivities for both polarisations reach an asymptotic limit as the number of layers increase. This is known as the Koppelmann limit and is illustrated in Figure B for a stack having a lossy high refractive index material with n = 2.3, and k = 5 x 10⁻⁵, a non-lossy low refractive index material with n = 1.46 and an angle of incidence of 45°. The graph of Figure B emphasises the necessity for using more layers when formulating a mirror for reflection in the p-polarisation mode if a particular value of reflectivity is required. The invention involves the use of a multilayer dielectric (MLD) reflecting stack comprising alternate layers of two dielectric materials having high and low refractive indices respectively and both having low absorption. To form a mirror each layer is made to have an optical thickness of a quarter wavelength of the radiation to be reflected and taking into account the angle of incidence of the radiation. As is well known, such an arrangement ensures that, for both p-and s-polarised light, the beams reflected from neighbouring layer interfaces reinforce and, if a sufficient number of layers is used, very high reflectivities can be obtained. A mirror would typically be made by depositing alternate layers of titanium dioxide (TiO₂) and silicon dioxide (SiO₂) onto a substrate of Zerodur. The initial and outermost layer being (TiO₂). Typical values for the refractive indices being 2.3 to 2.4 for the TiO₂ and 1.46 for the SiO₂ . The loss coefficients, the k of the complex refractive index (n-ik), being 5 to x 10 x 10⁻⁶ + 1 to 3 x 10⁻⁷ respectively. Generally such a mirror will have a half wave coating of SiO₂ on top of the outermost Tio₂ layer. This layer does nothing optically and is sometimes known as an absentee layer. Its purpose is to provide protection to the underlying TiO₂ layer from the environment. Such a layer will not be considered in the following discussion, it being understood that it can be added, if desired, to any of the designs considered. A method which has been proposed to produce a mirror which preferentially lases in the p-polarised mode involves the use of an anti-reflection coating which is polarisation sensitive (I.M. Minkov, Optics and Spectroscopy (USA), Vol. 33, No. 2, P175-178). The multilayer dielectric mirror so proposed comprises a repeat of a three layer anti-reflection coating which is more reflective for the p-polarised mode than for the s-polarised mode. However, the effect of absorption of light in the layers of such a mirror renders the proposal impractical. According to the present invention there is provided a mirror comprising a multi-layer dielectric reflective stack, alternate quarterwave layers for a specific wavelength of light at a p-polarised mode so that the transverse Kerr magneto-optic effect can be utilised to prevent the occurrence of lock-in of the counter-rotating beams. The above feature also makes use of the fact that the dielectrics are slightly absorbing and in particular those with a high dielectric constant generally have a higher absorption coefficient than those with a low dielectric constant. At optical frequencies, the dielectric constant of a material is approximately equal to the square of the refractive index, assuming a permeability of unity. To obtain a higher reflectivity for p-polarised radiation, the MLD design is modified so that the absorption is increased preferentially for the s-polarised component to such an extent that it reduces the level of the reflectivity so that it is less than that for the p-polarised component. For convenience the at least one layer may be positioned near the top of the multi-layer dielectric reflective stack. Alternatively, the at least one layer may be positioned remote from the top of the multi-layer dielectric reflective stack. The mirror may further comprise a metal layer underlying a control dielectric layer and the multi-layer dielectric reflective stack whilst the metal layer may be gold or silver. In a particularly advantageous arrangement, an off-tune multi-layer dielectric reflective stack underlying a control dielectric layer and the multi-layer dielectric stack may be provided. Advantageously, a magneto-optic layer is also provided. For a better understanding of the invention, reference will now be made, by way of example, to the accompanying drawings in which:- Figure 1 is a schematic diagram of a mirror according to the invention; Figure 2 is a graph of mirror loss against phase thickness for both p- and s-polarised light for the mirror of Figure 1; Figure 3 is a schematic diagram of a mirror according to a second embodiment of the present invention; Figure 4 is a graph of reflection coefficient against thickness of the control layer for the mirror of figure 3; Figure 5 is a schematic diagram of a mirror according to a third embodiment of the present invention; Figure 6 is a graph of mirror loss against wavelength for the mirror of figure 5; Figure 7 is a schematic diagram of a prior known mirror arrangement and; Figure 8 is a graph of mirror loss against wavelength for the mirror of figure 7. The mirror shown in figure 1 is in the form of a Fabry-Perot filter and comprises a substrate 1 on which a gold or silver film layer 2 is deposited. A control layer of dielectric material 3 is then formed on the film layer 2. On top of the control layer 3 is formed a multilayer dielectric stack 4 comprising alternate quarterwave layers made of dielectric film material. The lowest layer of the multilayer dielectric acts as a halfwave spacer while the rest of the layers make up the quarterwave reflecting stack as shown in Figure 1. For such a filter, the pass band ie . the transmission peak, is different for the p- and s-polarised modes (ignoring losses in the dielectric film layers of the stack 4) as shown in figure 2. Thus choosing the thickness of the control layer 3 can enhance transmission of s-polarised light, as indicated for example by the vertical dotted line in Figure 2, relative to that of p-polarised light thereby increasing the reflectivity of p-polarised light relative to s-polarised light. The filter of figure 1 has the main disadvantage that the thickness of the control layer 3 needs to be controlled very accurately. In the embodiment of Figure 3, the metallic layer 2 is replaced by a twenty layer multilayer dielectric which is off tune so that a phase shaft somewhere between 0° and 180° is obtained. The use of an off-tune multilayer dielectric imparts a polarisation dependence to the mirror arrangement. The stack of figure 3 comprises a substrate 5 on which is formed a twenty-layer multilayer dielectric 6 which is off tune. A variable thickness layer 7 is formed on top of this dielectric and comprises a layer of garnet. A fourteen-layer multilayer dielectric 8 is formed on top of the variable layer 7 - this layer is on tune. The reflection coefficient (R) plotted against the thickness of layer 7 is shown in figure 4 for both p- and s-polarisation modes. From the figure it can be seen that for an angle of incidence of 45°, the reflection coefficient for p-polarisation is a constant value whereas the reflectivity for s-polarisation drops off to produce a wave band around 0.65nm (6.5Å) where R s <R p . This arrangement also suffers from the practical difficulty of getting the pass band for s-polarisation on tune. Again Figure 4 does not include any compensation for absorption loss. When losses are considered, the width of the pass band of the Figure 3 stack is increased and makes the design more practical. It is possible to design a multilayer dielectric having R p >R s by making either the first or second layer of the stack of half-wavelength thick. A thirty-five layer stack is shown in figure 5 where the stack is formed by a multilayer dielectric 9, a halfwave layer 10 of silicon dioxide (SiO₂) and a quarterwave layer 11 of garnet all on a substrate 12. The response of this stack is shown in figure 6 and provides a more practical solution to producing a lasing cavity which preferentially lases in the p-polarised mode. It is thought that this embodiment works by enhancing the absorption of s-polarised light relative to that of p-polarised light. p and s-polarised light set up different types of standing wave in a MLD reflective stack. The standing wave for s-polarised light is of continuous relatively simply sinusoidal form whereas that for p-polarised light is of a discontinuous and complex form. In this embodiment according to the invention, this difference is exploited by arranging the mirror so that absorption is enhanced where this will most affect s-polarised light ie . at the intensity maxima of the s-polarised light standing wave. In practice it is found that maximising absorption of s-polarised light also helps to maximise the differential phase shift applied to counter-rotating laser beams when a suitable magneto-optic layer, such as a layer of garnet material, is included in the mirror. By providing less layers in the MLD reflecting stack a transmitting mirror can be produced as shown in Figure 7 in which the MLD 13 comprises twenty-seven layers mounted on a substrate 14. A half-wavelength dielectric layer 15 of high refractive index overlies the MLD 13, the top layer of which is a quarterwave layer of low refractive index The results obtained are shown in Figure 8 showing that Rp>Rs in the approximate waveband 620 to 645 nm. It has been found that using a half-wavelength layer further down in the MLD stack for a transmitting mirror enables a greater difference between the reflectivities of p- and s-polarised light to be obtained but over a relatively narrower waveband. The results referred to are obtained using a standard computer program for MLD devices. The results shown in figures 2, 4, 6 and 8 are for an angle of incidence of 45°, but naturally other angles of incidence are possible with corresponding modifications of the graphs to allow for changes in absorption in the dielectric materials used. An appendix follows which provides an analytical approach for a simple case showing the effect of incorporating a half-wavelength layer of low refractive index next to the top of an MLD reflective stack.

Claims: A mirror comprising a multi-layer dielectric reflective stack (4,8,9,13) comprising quarterwave layers for a specific wavelength of light at a predetermined angle of incidence, alternate layers of said stack having different refractive indices to each other, and at least one layer having an optical thickness of a half wavelength of the incident radiation at a predetermined angle of incidence, characterised in that said halfwave layer is not at the top of the stack, thereby enhancing the reflection of p-polarised light relative to that of s-polarised light. A mirror according to claim 1 further characterised in that said at least one layer is/are positioned near the top of the multi-layer dielectric reflective stack (4) A mirror according to claim 1 further characterised in that said at least one layer is/are positioned remote from the top of the multi-layer dielectric reflective stack (4, 8, 9, 13). A mirror according to any one of the preceding claims further characterised by the provision of a metal layer (2) underlying a control dielectric layer (3) and the multi-layer dielectric reflective stack (4). A mirror according to claim 5 further characterised in that the metal is gold or silver. A mirror according to any one of claims 1 to 4 further characterised by the provision of an off-tune multi-layer dielectric reflective stack (6) underlying a control dielectric layer (7) and the multi-layer dielectric stack (8). A mirror according to any one of the preceding claims further characterised by the provision of a magneto-optic layer. A ring laser gyroscope in which at least one of the mirrors provided therein defines a lasing cavity falling within the scope of any one of the preceding claims.

Inventors: CARTER WALTER SCOTT DR BRITISH, CARTER, WALTER SCOTT, DR. BRITISH AEROSPACE P.L.C.
Applicants: BRITISH AEROSPACE, BRITISH AEROSPACE PUBLIC LIMITED COMPANY
Application year: 1987
Main classification: G02B5
IPC codes: G01C19, H01S3, G02B5