Long-Term Photoreliability of Electro-Optic Polymers for High Optical Power Modulators

Introduction

Electro-optic (EO) polymers have been extensively researched for optical communication applications due to their outstanding features, including large EO coefficient (r33 > 100 pm/V), fast response, high responsiveness to electric fields, and compatibility with CMOS circuitry. These advantages enable high-speed and low-power consumption optical modulators. However, thermal stability and photostability have been major concerns for organic materials in practical applications.

While high Tg-EO polymers like side chains and cross-links have demonstrated excellent device performance for over 2,000 hours at 85°C, photostability of organic EO polymers remains challenging. Photodegradation occurs due to photooxidation of singlet oxygen caused by exposing organic materials to high-power lasers in the presence of oxygen, impacting the longevity and EO coefficient of EO polymer-based devices.

Strategies to Enhance Photostability

Several strategies have been suggested to enhance the photostability of devices employing organic materials, including:

1. Chemical stabilization of the EO chromophores

2. Addition of single-oxygen quenchers

3. Hermetically packaging the device in an inert atmosphere

The latter strategy has significantly improved the photostability of EO polymer devices. However, testing with high laser power under practical operating conditions for commercialized devices remains limited.

Experimental Setup

In this study, the preliminary photostability testing of a silicon organic hybrid (SOH) modulator employed EO polymer was conducted in an oxygen-free environment at an optical density of 0.74 mW μm-2. The EO polymer consisted of 35% FTC chromophore and highly thermal-stable adamantyl pendants, resulting in a Tg greater than 170°C.

The polymer solution was spin-coated on a Mach-Zehnder interferometer (MZI) waveguide of a modulator A (Fig. 1), where strong confinement of the optical field in the hybrid structure was achieved through the Si core and an EO polymer cladding. The device was placed inside an oxygen-free chamber at 80°C, and the 1550 nm-input laser was set to 10 mW and 40 mW. The Vπ was remeasured at time intervals over 2,000 hours at the maintained temperature.

Fig. 1. a) the cross sections of waveguide structure in each position of MZI modulator and b) their detailed description.

Results and Discussion

Fig. 2a depicts the comparison between photoreliability of a modulator A using low and high-power lasers in ambient atmosphere. The modulator was stable under low input power of 10 mW for over 500 hours. For 40 mW input power, the optical power in each arm of the MZI waveguide (position 2 in Fig. 1a) was 6.3 mW, resulting in an optical density on the EO polymer region of 0.74 mW μm-2. The presence of oxygen accelerated the degradation of the organic EO polymer, as Vπ quickly increased within a few hours of irradiation.

Fig. 2. Photoreliability test at 80°C in a) air, b) O2-free and c) argon of an modulator A using 40 mW input power

However, in an oxygen-free environment, the Vπ was substantially constant for more than 2,000 hours at high temperature (Fig. 2b). The experiment was also carried out in an argon atmosphere for more than 500 hours, yielding the same result (Fig. 2c). This demonstrates the excellent photoreliability of the EO polymer-based device in long-term scenarios when sealed in a hermetic package.

Conclusion and Future Outlook

The study demonstrated photostability of an EO polymer-based modulator at accelerating operating conditions for long-term testing. The device showed excellent stability under the irradiation of 0.74 mW μm-2 for 2,000 hours when in an oxygen-free environment, suggesting that sealing the SOH modulator in a hermetic package can achieve long-term photostability.

However, for long-haul optical communication, a modulator that can support higher optical power is needed. Therefore, it is important to test the EO modulator at higher power conditions. The researchers plan to use a strip-loaded slot waveguide based EO modulator, which is a promising candidate for highly confining the light (κ) within the slot approximately 50 times more effectively compared to the strip-loaded core design, as shown in Table 1. This enables larger phase shift using lower driving voltage (Fig. 1, Modulator B).

With such strong confinement, ensuring the device's photoreliability becomes more complex. Thus, photostability with actual optical density must be investigated to realize the reliability of the target device.

The researchers will perform the photoreliability test of the slot EO modulator at higher optical density in an oxygen-free atmosphere, carefully selecting the testing optical density while considering possible loss from two-photon absorption. This will enable them to assess the capability of an organic material to intense optical exposure in long-term use, similar to what it might experience when practically deployed.

Reference

[1] Bannaron, H. Sato, B. Ranjan, Y. Sakurai, M. Kawasugi, S. Yokoyama, "Long-term Photoreliability of Electro-optic Polymer for High Optical Power Modulator," Santec AOC Corporation, Aichi, Japan; Institute for Material Chemistry and Engineering, Kyushu University, Fukuoka, Japan, 2024, pp. 1-6, doi: 979-8-3503-9404-7/24/$31.00 ©2024 IEEE.