The present invention relates to boundary surfaces between optical media with different refractive indices in measuring devices.
In some optical applications, in particular in fiber optical paths, it is important to avoid or at least reduce unwanted return reflections, i.e. reflections in the direction of an incident optical signal, since such return reflections can disturb e.g. the source (for example a laser diode) of the optical signal. This can lead, amongst others, to signal deviations of the optical signal source.
Reflections, in general, are caused by boundary surfaces (also referred to as contact surfaces) in the optical path due to differences in refractive indices. E.g. in fiber optical paths, reflections are caused at all boundary surfaces and in particular at fiber terminations, such as fiber connections. High values of reflections result in particular from transitions between the optical path media and air for transitions perpendicular to the propagation direction of the optical path. Assuming an optical glass fiber with a refractive index of n=15 would result in a reflectivity R=4% for such a perpendicular glass air transition. Using the definition of return loss RL in logarithmic dB values:
RL=xe2x88x9210 lg(R)
leads to a return loss of RL=14 dB for the exemplary perpendicular glass-air transition.
In order to diminish the influence of unwanted reflections, boundary surfaces such as optical path terminations are generally provided to be angular, angled or tilted (referred to in the following as xe2x80x98angularxe2x80x99). In optical fibers, such fiber terminations are usually grinded or cut in an angular manner. Typical angles of a standard single mode fiber with approximately 0.008 to 0.010 mm mode field diameter (1300 nm, 1550 nm) are about 8xc2x0. The return loss RL in such a case will be greater than 60 dB. U.S. Pat. No. 5,574,809 discloses an optical fiber with inclined end faces with anti-reflection films formed thereon.
Disadvantageous in providing angular terminations of the optical path, however, is the polarization dependent transmission through that termination which is caused by that angular boundary surface. Since optical signals transmitted on optical paths, such as fibers, are normally polarized, transmission and accordingly loss at such angular boundary surfaces (e.g. fiber-fiber or fiber-air) is dependent on the state of polarization of the optical signal. The state of polarization of the optical signal, however, will be modified by any deflection and/or change in temperature and fiber bending within the optical path, so that the state of polarization at the angular boundary surfaces is undefined and varies statistically. The optical power of the optical signal will thus be modified in a statistical and accordingly undefined manner at each boundary surface with polarization dependent transmission characteristics. This effect even increases if there are a plurality of polarization dependent boundary surfaces located within the optical path.
The polarization dependent loss (PDL) is generally defined as:   PDL  =                    -        10            ⁢              xe2x80x83            ⁢              lg        ⁡                  (                                    Δ              ⁢                              xe2x80x83                            ⁢              P                                      P              average                                )                      ≅                  -        10            ⁢              xe2x80x83            ⁢              lg        (                                            P              max                        -                          P              min                                                          1              2                        ·                          (                                                P                  max                                +                                  P                  min                                            )                                      )            
whereby xcex94P represents the difference in power between a maximum power value Pmax and a minimum power value Pmin of a back reflected signal under the influence of polarization which might occur for an incident signal. Paverage represents the average power that can be approximated as xc2xd(Pmaxxe2x88x92Pmin).
U.S. Pat. No. 4,492,436 and Mordechai Gilo, Design of a nonpolarizing beamsplitter inside a glass cube, Applied Optics, Sep. 1, 1992, Vol. 31, No. 25, pages 5345-5349 disclose polarization independent beam splitters. In Gilo, the transmittance values are designed and optimized to give either Tp=Ts or Tp+Ts=constant, in the vicinity of xcex0. A polarization independent, linear tuned interference filter with constant transmission characteristics is disclosed under this title by N. Mekada et al., IEEE Photonics Technology letters, June 1997, vol. 9, No. 6.
If measuring devices are coupled to the optical path for determining the optical power of the optical signal, the measuring results are modulated by the statistically modifying polarization dependent loss, e.g. at the boundary surface towards the measuring device, thus increasing the (rated) measuring fault. In the above-mentioned example of a tilted termination boundary surface with an angle of 8xc2x0 and a transition between glass (refractive index of 1.5) and air, the measuring fault will be approximately 0.5% or 0.022 dB (peak to peak).
It is an object of the present invention to reduce the influence of return loss (RL) and polarization dependent loss (PDL) on boundary surfaces in an optical path for measuring purposes. The object is solved by the independent claim. Preferred embodiments are shown by the dependent claims.
According to the invention, a boundary surface (between optical media with different refractive indices) of a measuring device is provided to be angular in order to reduce back reflection of an optical signal at that boundary surface into the direction of the source of the optical signal.
The boundary surface according to the invention further provides a transmission (through the boundary surface) independent of the state of polarization of the incident optical signal. This can be achieved in that the transmissions of the optical signal perpendicular and parallel to the plane of incidence of the angular boundary surface are substantially equal. The plane of incidence is generally defined by the incident signal beam and the normal (or plumb line) to the boundary surface. Thus, the boundary surface provides an interface between optical media with different refractive indices, whereby polarization dependent effects at the boundary surface can be reduced or even be avoided since the transmission through the boundary surface becomes independent of the state of polarization of the incident optical signal. This substantial independence of the state of polarization of the incident optical signal, however, generally leads to a substantially constant loss or the optical signal at the boundary surface, which is independent of the state of polarization.
The invention thus, on one hand, leads to a reduction of the return reflections towards the incident optical signal and, on the other hand, leads to a substantially constant loss substantially independent of the state of polarization of the incident optical signal.
For avoiding or reducing the polarization dependent loss (PDL) the angular boundary surface (e.g. a termination facet) will be preferably provided with a specific coating substantially fulfilling the condition:
Ts=Tp
whereby Ts is the transmission perpendicular to the plane of incidence of the boundary surface, and Tp is the transmission parallel to the plane of incidence of the boundary surface. For physical reasons, the transmission rates of the optical signal perpendicular and parallel to the plane of incidence of the angular boundary surface will generally be smaller than 100%.
In case that Ts=Tp, there is no polarization dependent loss at the boundary surface even though the boundary surface is angular.
A coating of the boundary surface is preferably provided to achieve a polarization independent transmission and also to reduce reflection at the boundary surface.
In the ideal case, the boundary surface should provide a smooth and continuous gradient between the refractive indices. This, however, encounters the problem of technical feasibility. Instead of such a continuous gradient between the refractive indices, a plurality of individual layers can be provided e.g. by evaporating or sputtering processes. It is to be understood that a plurality of individual layers also provides more degrees of freedom for the optical design. For technical or cost reasons it might be necessary to limit the number of materials applied for the individual layers. In such a case, it has been found that a sequence of alternating layers of only two different materials already exhibited remarkable results.
General properties of applicable materials for the polarization independent boundary surface are that the material can be applied as preferably thin layers and the material preferably provides dielectric, loss-free layers. Suitable materials have been found in the following list of common evaporating materials:
hafnium oxide HfO2 
titanium oxide TiO2 
aluminum oxide Al2O3 
zirconium oxide ZrO2 
silicon oxide SiO2 
When designing the plurality of individual layers, in particular the interference properties of the layers, alone and in combination with each other, have to be taken into account preferably in order to reduce back reflected light as much as possible.
The polarization independent coating of the boundary surface is preferably provided to be also dereflective or anti-reflective, thus further reducing return reflections. By carefully selecting coating parameters, such as number of coating layers, coating materials and/or layer thickness of the return reflection reducing coating, the above given condition Ts=Tp can also be fulfilled for coatings wherein the average transmission is smaller than 100%, or, in other words, for coatings which provide the condition:             Ts      +      Tp        2     less than       100    ⁢          %      .      
That means that even xe2x80x98non-perfectxe2x80x99 dereflective coatings according to the invention with reflectivity values of R greater than 0 do not provide polarization dependent transmission losses and can thus be applied.
In case that the values of the perpendicular and parallel transmission Ts and Tp (respectively) exhibit dependencies on the wavelengths of the incident optical signal, a certain tradeoff or compromise for different wavelengths or one or more wavelength bands might have to be made in order to reduce the polarization dependent loss for those wavelengths or wavelength bands. In such a case, the condition Ts=Tp will hardly be fulfilable for all wavelengths so that a certain compromise has to be made and a certain wavelength dependent polarization dependent loss will have to be encountered. However, by carefully balancing Ts(xcex) and Tp(xcex) the affect of polarization dependent loss can be efficiently minimized for certain wavelengths or wavelength bands of interest.
The invention is preferably used for providing a defined termination (facet) of an optical path, in particular of an optical fiber path but can be employed for any kind of boundary or contact surface. Such defined terminations of optical paths are in particular useful for measurement purposes since the measurement fault, and thus the measurement resolution, can be significantly improved by eliminating or at least reducing polarization dependent effects.
The invention can be applied e.g. for termination surfaces, termination adapters, detector surfaces, or the like.