he next phase in integrated optics spins around using the characteristics of magneto-optics.

Magneto-optics is uniquely enabling for the production of nonreciprocal components such as optical isolators and circulators. The concepts behind the nonreciprocity include polarization rotation (Faraday rotation), nonreciprocal phase shift and guided-mode-to-radiation-mode conversion. A magneto-optic material, magnetized in the direction of propagation of light, acts as a Faraday rotator. When a magnetic field is applied transversely to the direction of light propagation in an optical waveguide, a nonreciprocal phase shift occurs and can be used in an interferometric configuration to result in unidirectional propagation. Nonreciprocal guided-mode-to-radiation-mode conversion has also been demonstrated.

Today, commercial isolators and circulators are strictly bulk components; as such they constitute the only type of optical component that is not available in integrated form. However, the technology for integrated nonreciprocal devices has been maturing and is expected to have a considerable impact in the communication industry by enabling the integration of complete optical subsystems. Here we review the progress.

Optical isolators are two-port unidirectional devices that transmit light in one direction and block it in the opposite direction. They are used to eliminate light that is back-reflected into lasers in optical telecommunication systems.

Optical isolators must be integrated because it reduces component size and cost, simplifies alignment (i.e., no collimating lenses, etc.), enables mechanical and thermal stability, requires a smaller saturation magnetic field (a few to tens of Oe) and it provides mode matching to other waveguide devices (i.e., lasers, modulators, etc.), enabling high levels of integration with low insertion loss. Table 1 summarizes the work done to date in integrated optical isolators.

Two routes exist to integrated optical isolators and circulators: hybrid and fully integrated. Hybrid Faraday rotation optical isolators require hybrid integration and alignment, integration of film magnets, hybrid polarizing elements and magneto-optical materials.

To date a number of developments are worth noting:

N. Sugimoto et al.[1] produced a hybrid optical isolator on a silica planar lightwave circuit (PLC). The implementation of that device required alignment between the waveguide Faraday rotator and silica waveguide and an external magnet. This device exhibited high isolation (~30 dB) with low total insertion loss (~3 dB).

Levy et al.[2] demonstrated a Bi-YIG (bismuth-substituted yttrium iron garnet) Faraday rotator with a thin-film SmCo (samarium cobalt) magnet. That device achieved magnetization saturation by a thin-film magnet sputtered on an epitaxial Bi-YIG waveguide structure. It resulted in up to 29-dB isolation ratio with negligible excess loss and an absorption loss below 1 dB.

Thin-film half-wave plates are necessary to bring 45 deg. polarization into TE or TM mode polarization. Inoue et al.[3] have produced 14-m-thick polyimide half-wave plates with more than 20-dB conversion ratio, less than 0.3-dB excess loss at 1.50 to 1.57 m, but the material is hygroscopic. Radojevic et al.[4] demonstrated 10-m-thick crystal ion sliced (CIS) LiNbO3 half-wave plates, with more than 33-dB conversion ratio and less than 0.1-dB excess loss at 1.47 to 1.57 m, and the material is not hygroscopic.

Faraday rotation can be enhanced in photonic bandgap structures consisting of magnetic thin film stacks.[5] To avoid TE/TM mismatch, the waveguide rotator can be replaced with unguided propagation.

A simplistic view in magnetic thin film stacks is that all one has to do is trap light in a cavity and the rotation will be enhanced. Although this concept is true, the transmission that results is unacceptably reduced. One solution is to use interactions of defect splitting and magnetic splitting to create accidental resonances and achieve any rotation with a full transmission. This is a narrowband solution.

One-dimensional photonic crystals in cerium-substituted yttrium iron garnet (Ce-YIG) with multiple defects were studied at 1.55-m wavelength and it was found that the interdefect spacing can be adjusted to yield a flat top broadband response, with close to 100 percent transmission and 45-degree Faraday rotation.[6] The length in a four-defect structure is only 36 m, as compared to 120 m for the best Ce-YIG films. A linear relation between number of defects and bandwidth was found; it shows a transmission band that is 2.4, 4.6, 6.3, 8.2 and 10.4 nm wide for, respectively, three, four, five, six and seven defects.

In summary, hybrid integration of Faraday rotators and polarizing elements has been demonstrated, thin-film magnets have been integrated on waveguide Faraday rotators, a major advance has been achieved by inserting film half-wave plates and extinction ratios of about 30 dB have been obtained for a wide range of wavelengths around 1.55 m, comparable to commercially available bulk isolators. However, birefringence control is still needed and the total insertion loss needs improvement (lowest value to date is 2.6 dB).

A major difficulty with Faraday-rotator-based optical isolators is that phase matching has to be achieved for efficient isolations, but it is typically prevented by spurious birefringence. One solution involves avoiding phase matching by using a Mach-Zehnder-interferometer-based isolator, which utilizes a transverse magnetic field to create a nonreciprocal phase shift.[7]

The advantages of Mach-Zehnder isolators include no phase-matching requirement, averting fabrication-induced birefringence, reducing alignment, the possibility of not needing an integrated polarizer and the potential for polarization-independent isolators.

The technologies needed include low-loss, high Faraday-rotation materials; characterization methods for magneto-optical materials; fabrication methods; vectorial numerical simulation and design; thin-film magnets and integration of isolators with host waveguides (wafer bonding), a major fabrication challenge.

High-performance optical isolators based on Mach-Zehnder interferometers have been demonstrated; needed now is improved materials.

A need exists for TE and polarization-independent isolators, since many laser sources operate in the TE mode and because components in fiber-optic communication systems require polarization independence. TE-mode and polarization-independent isolators have been proposed based on Mach-Zehnder interferometry by Zhuromskyy et al. and Fujita et al.[9,10] TE and TM nonreciprocal phase shifts has been observed by both groups.

A new integration materials technology has been developed for optical isolators and it involves direct wafer bonding for on-chip optical isolators.[11]

Film magnets have been integrated on Mach-Zehnder interferometers.[12] About 1-m-thick SmCo is enough to create opposing magnetic field and saturate iron garnets.

Recent Mach-Zehnder interferometer technologies include waveguide Mach-Zehnder isolators with transverse magnetic field exhibiting extinction ratios of around 20 dB, the integration of film magnet and transfer of isolators on other materials (such as InP) and the emergence of low-loss (less than 1dB/cm) high-performance (required arm length ~1mm) magneto-optical materials.[13] Mach-Zehnder isolators are a promising route to achieve integrated-optical isolation.

An integrated optical isolator based on nonreciprocal guided-mode-to-radiation-mode conversion has also been demonstrated.[14] It utilizes efficient nonreciprocal conversion from a fundamental TM mode to a deep TE radiation mode away from the cutoff. The isolator was realized using a single-mode rib channel waveguide in Ce-YIG. An isolation of 27 dB was obtained.

Optical circulators are multiport unidirectional devices that route light circularly from port to port. In an N-port circulator, light is routed from Port 1 to Port 2, from Port 2 to Port 3, ..., from Port N-1 to Port N and from Port N to Port 1. They are used in routing systems that involve reflective elements, such as in optical add/drop multiplexers based on Bragg gratings.

Integrated optical circulators use the same basic nonreciprocal concepts as integrated isolators. Different architectures have been proposed. Fig. 1 shows a polarization-independent design.[17] It includes a planar Mach-Zehnder interferometer where the arms are magneto-optically active in the presence of a permanent magnet and two half-wave plates that enable polarization independent operation. The basic configuration and principle of operation are shown in Fig. 1(a) and the PLC implementation is shown schematically in Fig. 1(b).

References

[1] N. Sugimoto et al., J. Lightwave Technol., vol. 14, pp. 2537-46 (1996).

[2] M. Levy et al., Photon. Technol. Lett., vol. 8, pp. 903-5 (1996).

[3] Inoue et al., J. Lightwave Technol., vol. 15, pp. 1947-57 (1997).

[4] T. Radojevic et al., Photon. Technol. Lett., vol. 12, pp. 1653-5 (2000).

[5] M. Steel et al., Photon. Technol. Lett., vol. 12, pp. 1171-3 (2000).

[6] M. Levy et al., unpublished.

[7] J. Fujita et al., Appl. Phys. Lett., vol. 75, pp. 998-1000 (1999).

[8] J. Fujita et al., Appl. Phys. Lett., vol. 76, pp. 2158-60 (2000).

[9] O. Zhuromskyy et al., J. Lightwave Technol., vol. 17, pp. 1200-5 (1999).

[10] J. Fujita et al., Photon. Technol. Lett., vol. 12, pp. 1510-2 (2000).

[11] T. Izuhara et al., Appl. Phys. Lett., vol. 76, pp. 1261-3 (2000).

[12] M. Levy et al., LEOS ‘96, pp. 232-3 (1996).

[13] L. Wilkens et al., unpublished.

[14] T. Shintaku, Appl. Phys. Lett., vol. 73, p. 1946-8 (1998).

[15] Casix Inc., Technical product specification sheet.

[16] L. Wang et al., Chinese J. of Lasers B, vol. 8, p. 1 (1999).

[17] N. Sugimoto et al., Photon. Technol. Lett., vol. 11, pp. 355-7 (1999).