Microlens fiber coupling enhances performance of EDFAs
Improving the beam-coupling efficiency between pump laser diode and optical fiber is critical to EDFAs. A cylindrical meniscus microlens can correct for beam circularity and astigmatism in a single step and be integrated with the diode in a compact package.
Maximizing the efficiency of DWDM systems using erbium-doped fiber amplifiers (EDFAs) requires careful attention to the coupling between the EDFA pump laser diode (usually a 980-nm diode) and the optical fiber itself. A less-than-optimal coupling can be a significant source of energy loss and signal degradation, leading directly to increased energy consumption and higher overall system costs. Furthermore, optimizing the coupling efficiency can also ease manufacturing tolerances, resulting in lower manufacturing costs per device.
Typical edge-emitting laser diodes used as pump lasers in EDFAs are imperfect light sources because their emitting aperture is rectangular rather than square or circular, resulting in a laser output that is elliptical in cross section. Since maximum coupling efficiency requires accurate mode matching, a noncircular beam will always result in less-than-maximum fiber-coupling efficiency. In addition, because the emitting source-points of the fast and slow axes are offset, the laser diode is inherently astigmatic, which degrades the focus and reduces coupling efficiency. Finally, the fast axis may have a divergence that exceeds the numerical aperture of the focusing optics, resulting in clipping of the diode's output beam.
Fiber coupling using correcting optics can compensate for the deficiencies inherent in edge-emitting laser diodes. A cylindrical meniscus microlens can be used to correct for circularity and astigmatism in a single step, with a lens that can be integrated with the diode in a compact, sealed package (see Fig. 1). The first surface of the microlens corrects beam divergence to match fast and slow axes, correcting for ellipticity, while the second surface further modifies the beam to bring the point sources for the two axes to a single "virtual point source," correcting for astigmatism. The corrected beam not only allows maximum coupling efficiency but also leads to increased fiber alignment tolerance, which translates to greater manufacturing yields and lower cost per device for EDFAs and other laser-diode-based components.
In a cylindrical microlens, the first curved surface of the cylindrical lens slows down the fast-axis divergence such that, at the second curved surface of the lens, the beam size of the fast-axis light is equal to the beam size of the slow-axis light. The slow-axis divergence is unaffected. The second curved surface of the cylindrical lens further modifies the fast-axis divergence to match the slow-axis divergence (while the slow-axis divergence is again unaffected), bringing the fast-axis virtual source coincident with the slow-axis virtual source, forming a single virtual point source (VPS) for both the fast and slow axes (see Fig. 2).
In this example, the aspect ratio (fast-axis divergence/slow-axis divergence) of the beam profile is 2.4 before correction, while after the correction the aspect ratio of the beam profile is 1.0 and the resulting beam is circular. Also, before the correction, the fast-axis source and the slow-axis source are separated by 4 µm. After the correction, the two sources are located at the same VPS position, which reduces the astigmatism to zero.
After the laser-diode beam is corrected for circularity and astigmatism, it can be coupled with a fiber with high efficiency. Since the coupling efficiency depends on the mode matching in the single-mode fiber and the mode field of fiber is circular, an elliptical beam can never achieve the maximum coupling efficiency. Thus, without using a cylindrical microlens, the coupling efficiency will be low. After the laser beam is corrected by the microlens, it becomes a circular Gaussian beam with no astigmatism. The coupling efficiency of this regular Gaussian beam and a single-mode fiber can achieve the maximum theoretical coupling efficiency.
In addition, optimizing the efficiency of coupling a typical 980-nm pump laser diode with a single-mode fiber requires a coupling lens to match beam waists (diameters). For example, in Fig. 2 the beam full-width, half-maximum (FWHM) divergence is 5°, with a corresponding Gaussian beam waist of approximately 2.1 µm.
Since the mode-field radius of the fiber is 3.1 µm, to achieve the maximum coupling efficiency the Gaussian beam waist of laser light must be magnified by 3.1/2.1. To achieve this, the elliptical laser beam can first be corrected by a cylindrical microlens, and then launched into the single-mode fiber by a coupling lens. The microlens changes the initially elliptical beam to a circular beam and corrects astigmatism, while the coupling lens matches the Gaussian beam waist with the fiber mode-field radius, thereby maximizing coupling efficiency (see Fig. 3).
The microlens coupling also allows relaxed fiber-alignment tolerances, which simplifies manufacturing and allows the use of robotics. In conventional applications, the laser diode is directly butt coupled with a tapered fiber or lensed fiber. In addition to low coupling efficiency, the tolerance of displacement d in the case of butt coupling is also smaller than that of the lens coupling.
The transmission T (in percent transmitted) resulting from the transversal displacement of the fiber d is given by,
where w is the mode-field radius and d is the tolerance of displacement. The value of T is calculated for the case of microlens coupling (and the example 980-nm diode) using the actual mode-field radius w of the fiber, which is 3.1 µm. Conversely, for butt coupling, T is calculated with w given by the Gaussian beam waist of laser, which can be calculated using either 2.1 µm for the slow axis or 0.9 µm for the fast axis.
For the typical 980-nm pump laser diode using a microlens coupling, a transmission of 85% results with a fiber displacement d of 1.25 µm (mode-field radius w = 3.1 µm). To achieve the same 85% transmission efficiency with butt coupling, the fiber displacement d is limited to from 0.85 µm (for w = 2.1 µm for the fast axis) to as little as 0.36 µm (for w = 0.9 µm for the slow axis).
Since the fiber-alignment tolerance for coupling can be relaxed when a microlens is used, manufacturing yields increase while the costs per device decrease. Laser-diode-based components can then be produced at much lower cost than by conventional methods. Furthermore, the microlens can be produced with very high entrant numerical apertures, capturing nearly all of the diode power. By providing for higher fiber-coupling efficiency at lower cost, microlens-based laser-diode components will continue to decrease the cost of DWDM systems while at the same time enhancing system performance.
Suganda Jutamulia is a senior member of the technical staff at Blue Sky Research, 1537 Centre Pointe Drive, Milpitas, CA 95035. He can be reached at firstname.lastname@example.org.