**4. Conclusion**

302 Holograms – Recording Materials and Applications

( 1) (1 )

( 1) ( 1)

These equations are still valid for port 1 and port 2 to determine the position of its associated RP. Shown in Fig. 18 is a 5-port polarization-independent optical quasi-circulator consisting of a pair of HSPMx and HSPMy, 3 PBSs, and 8 RPs. Figures 18(a) and 18(b) show the routes of port 1→port 2 and port 4→port 5, respectively. Based on the same principle,

(a) (b) Fig. 18. Structure and operation principles of the proposed 5-port polarization-independent

A 5-port polarization-independent optical quasi-circulator for 1300nm can be assembled with the fabricated HSWPs. In addition to a pair of orthogonal HSPMs, it needs another three PBSs and eight RPs to complete the function of this device. Its associated losses and isolation values are estimated and listed in Table 5. The isolation values are in the range from 20 to 54dB. The return losses and the insertion losses are about 14dB and about 3dB, respectively. Return losses mainly come from the interface reflections that influence the isolation values directly. If the applied HSWPs are anti-reflection coated and are fabricated under accurate fabrication processes, the return losses could be over 50 dB and the diffraction efficiencies may reach theoretical values, i.e., *η*h=0% and *η*v=100%. Under these two improved conditions, the performance of this 5-port optical quasi-circulator can be enhanced greatly with isolation values larger than 51dB and insertion losses smaller than

*PBS n PBS n z L z L*

*PBS n PBS n z L z L*

*x y nL nL* <sup>=</sup> <sup>=</sup>

− − =− =−

*x y n L nL x y nL n L x y n Ln L*

− − =− −

− −

*x y nL n L x y n L nL*

<sup>−</sup> = −

other propagation and expanded routes can also be obtained.

(1 ) ( 1) ,

(2 ) (2 ) , (for an odd port) (21)

(for an even port) (22)

1(2 1) 1(2 1) 2(2 1) 2(2 1) (2 1) (2 1)

− − − −

1(2 ) 1(2 ) 2(2 ) 2(2 ) (2 ) (2 )

*RP n RP n RP n RP n*

*RP n RP n RP n RP n*

optical quasi-circulator.

0.9dB.

In this chapter, polarization-selective substrate-mode volume holograms were introduced which are applied in several novel designs of optical circulator. Alternative design method of polarization-selective substrate-mode volume holograms was also introduced for overcoming the finite refractive index limit in practical holographic recording materials. The described optical circulators have advantages of polarization-independent, compactness, high isolation, low polarization mode dispersion, easy fabrication, and low cost. In addition, the port number of the proposed multi-port device can be expanded easily. High application potential of these devices in optical communications is expected. Their commercialization finally relies on available high-performance holographic recording materials.
