Evanescent Coupling Lights up a Fiber

Breck Hitz

A fundamental problem of modern photonics is transferring light efficiently between the optical fibers that transport light and the semiconductor devices that generate, control and detect it. One promising, albeit nascent, approach is to grow the semiconductors inside optical fibers (see “Building Semiconductor Structures in Optical Fiber,” Photonics Spectra, August 2006, page 80). Alternatively, scientists at Stanford University in California have recently shown that evanescent coupling between a specially designed semiconductor laser and an optical fiber can be an effective mechanism.

Commercially, the commonly accepted method uses free-space optics to get light from a semiconductor device into an optical fiber, or vice versa. The technique of contacting a fiber against, or nearly against, a semiconductor is inelegantly termed “butt-coupling” and is widely employed. But it is expensive because alignment between the two components is critical, and it is inefficient because the optical modes of the two rarely match.

The evanescent approach developed at Stanford, in contrast, can be cheap to fabricate because alignment is simple, as well as efficient and stable, because the laser waveguide resonates evanescently with the optical fiber. Moreover, because the coupling scheme relies on a narrow-bandwidth resonator formed partly by the semiconductor structure and partly by the fiber, the laser oscillates in a stable single mode.

Figure 1. Evanescent coupling occurred when the side-polished fiber was pressed against the antiresonant reflecting optical waveguide (ARROW) structure of the semiconductor. The integrated ARROW-fiber-laser resonator was formed in the waveguides between the metallic mirror deposited on the end of the fiber and the cleaved facet of the ARROW. Images reprinted with permission of Applied Physics Letters.

Although the large index contrast between conventional semiconductor lasers and silica fibers discourages evanescent coupling, an unusual semiconductor waveguide can propagate indices low enough to allow it. An antiresonant reflecting optical waveguide — known by its acronym ARROW — is a one-dimensional photonic bandgap structure whose intracavity rays can travel at an arbitrary angle relative to the distributed Bragg reflector mirrors above and below the gain region. By designing the system properly, the researchers formed an integrated fiber-semiconductor laser that evanescently coupled light at the resonant wavelength from the AR-ROW structure directly into the fiber.

Figure 2. The researchers obtained up to 3 mW of output in a single mode stabilized by the resonant coupling between the ARROW and the fiber.

The experimental device brought a side-polished, single-mode fiber into direct contact with the top surface of the GaAs ARROW structure (Figure 1). The laser resonator is effectively formed between the metallic mirror deposited on the end of the fiber and the cleaved facet of the semiconductor. Because the distributed Bragg reflector mirrors form a waveguide whose rays propagate at an oblique angle and hit the cleaved facet at an angle greater than the critical angle, the reflection from that surface is total. The researchers etched the facet at the other end of the ARROW structure at an angle to avoid modes oscillating between the two facets.

Figure 3. Evanescent coupling minimizes the loss that occurs between a diode laser and an optical fiber.

They electrically pumped the laser with a pulsed current (0.5-μs pulse width at a 1-kHz rate) to avoid overheating it, and observed up to 3 mW output in a clean and stable single mode (Figure 2). The resonant coupling scheme between the ARROW and the fiber ensured that the spectrum had no side modes within 27 dB of the prime wavelength. Because the lasing wavelength depended only on the refractive indices, it was temperature-stable with a variation of only ∼0.06 nm/°C.

Applied Physics Letters, 2006, Vol. 89, 041105.