In a novel one-step process, a vertical-cavity surface-emitting laser (VCSEL, operation wavelength of 980 nm) is integrated with a hybrid microdiffractive lens by focused ion beam milling (FIBM) for use in free-space optical links. A hybrid microlens with a diameter of 100 μm, numerical aperture of 0.56, and sag height of 4.196 μm, combined with a diffractive lens with continuous relief and 6 annuli, was designed and fabricated in one step by FIBM on the back of a VCSEL with a GaAs substrate for beam collimation. A previous VCSEL integrated with a pure diffractive lens had a half-divergence angle of 0.6°; the half-divergence angle of the VCSEL with the hybrid microlens was improved to 0.3°. Test results show that athermalization with respect to the variation in operating temperature can be realized with the hybrid microlens.
© 2002 Optical Society of America
Vertical-cavity surface-emitting lasers (VCSELs) hold great potential for data transmission in free-space optical interconnections. A VCSEL integrated with a pure diffractive lens was introduced previously [1, 2]. However, integrated pure diffractive lenses are still limited by high dispersion and difficulties in achromatization and thermal compensation. For example, it is difficult to ensure the stable coupling of light output to fiber or to a photodetector for a VCSEL operating at an ambient temperature between 20°C and 100°C. In this paper we present a hybrid microlens that can replace the previous pure diffractive lens to be integrated with the VCSEL operating in single-mode for beam collimation.
Compared with the pure diffractive lens, the hybrid microlens (shown in Fig.1) has the following advantages:
- The N.A. can be higher than in a traditional refractive lens;
- Athermalization is possible;
- Material requirements are less stringent;
- The high dispersion caused by a pure diffractive lens can be avoided.
The third advantage is important for free-space optical interconnection packaging. If we set the thermal expansion coefficient of the hybrid microlens equal to that of the material of the flip chip solder bumps, athermalization of the hybrid microlens is possible because of its inherent optothermic performance. For a normal VCSEL with an operation temperature ranging from 20°C to 100°C, athermalization becomes important. Because in practical operation a VCSEL is always used in an array, so that the junction temperature is higher than it would be in a single VCSEL, thermal compensation of the optical system becomes an especially critical issue [3–5].
2. Design and fabrication of the hybrid microlens
The single-mode bottom-emitting VCSEL used in the experiments was a conventionally designed device with an operation wavelength of 980 nm and oxide aperture diameter of 8 μm, built as an index-guided device. The top (bottom), p- (n-) type Bragg mirror consisted of 20 (30) periods of alternate AlGaAs–AlAs layers. Under continuous-wave operation (with no heat sink) the device showed threshold currents (voltage) of 0.74±0.02 mA (1.44±0.01 V), operating current (voltage) of 3 mA (2.5 V). It operates in TEM00 mode up to an output power of ∼4 mW.
Athermalization is the most apparent advantage of the hybrid lens. Temperature compensation can be realized by use of this characteristic. The refractive and diffractive power of the hybrid lens can be determined by solution of Eq. (1) :
where ϕ is the designed optical power, ϕr the refractive optical power, ϕd the diffractive optical power, xf the thermal expansion coefficient of the hybrid lens (ppm/°C), xf,r the thermal expansion coefficient of the refractive lens (ppm/°C), and xf,d the thermal expansion coefficient of the diffractive lens (ppm/°C).
For a refractive lens, thermal behavior is wavelength dependent. For a diffractive lens, the change in focal length is a function only of the thermal expansion coefficient . It is not a function of thermally induced changes in the refractive index of the lens material. Therefore athermalization does not require the integrated optical system to have a low optothermic expansion coefficient; rather, the optothermic expansion coefficient of the optical system should be matched to the thermal expansion of the packaging material in the free-space optical interconnection system.
For a working wavelength of 980 nm and a GaAs substrate, the optothermic expansion coefficient of the refractive lens, xf,r, and the optothermic expansion coefficient of the diffractive lens, xf,d, are -59.05 ppm/°C and 56.6 ppm/°C, respectively.
where λ is the working wavelength of the VCSEL, n the refractive index of GaAs (3.3 for λ = 980 nm), ω 0 the half-aperture diameter of the VCSEL (here 4 μm), and ω 1 the beam half-diameter at the hybrid microlens, as shown in Fig. 1.
The relationship among the beam diameters at the site of the hybrid microlens, the beam waist, the distance in space, and the receiver can be seen in Refs.  and . We assumed that the material of the flip chip solder bump packaging between the optical system and the backplane of the receiver is gold. We set the optothermic expansion coefficient of the hybrid lens to be xf= αAu = 25.6 ppm/°C (the thermal expansion coefficient of a gold solder bump). For the defined focal length of the diffractive lens and refractive lens, the focal length of the hybrid micro lens can be calculated by using Eq. (3) in terms of the optothermic characteristics—the athermalization of the hybrid microlens :
where f is the focal length of the hybrid microlens, fd the focal length of the diffractive lens, and fr the focal length of the refractive lens.
The lens is designed with the parameters shown in Table 1, based on the above considerations. The fabrication process is described in detail in Refs.  and . It takes 30 min to fabricate a single lens. Alignment of the hybrid microlens on the GaAs substrate with the VCSEL aperture can be carried out during FIBM . In this research the following parameters were used: an interline step of 0.5 μm, a beam spot size of 100 nm, and an ion-beam overlap of 60%. Figure 3(a) shows a three-dimensional atomic force microscope (AFM) measurement of the hybrid microlens fabricated by FIBM technology. Figure 3(b) shows the hybrid microlens profile measured by AFM with a measurement accuracy of ±0.15 μm within the range of 30 μm under fine calibration. It can be seen that the pattern is neat and symmetric and that the relief surface has a mirror finish. The measured diameter of the hybrid microlens is 100.84 μm. The surface rms roughness of the hybrid microlens over a relief area of 1 μm × 1 μm is 1.5 nm.
3. Results and Discussion
The FIBM process has little effect on the VCSEL performance [2, 5]. The beam divergence strongly depends on the driving current. This results in a variation of the numerical aperture (N.A.) of the output beam from 0.10 to 0.22, increasing with the driving current . The beam spot diameter was measured with a beam scanner (BeamScope-P5), and the VCSEL was set to operate with a drive current of 3 mA. Divergence angles for the VCSEL without the integrated microlens, with a diffractive lens, and with the hybrid microlens, calculated in terms of the inclination angle of the lines as shown in Fig. 4, are 9.5°, 0.6°, and 0.3°, respectively. Figure 4 reveals that the lens aberration is decreased and the output beam is stable. Figure 5 is a far-field image of the VCSEL emitting beam spot, with the integrated hybrid microlens at a distance of 65 μm from the emitting surface to the CCD array plane.
After system packaging by the flip chip technique with a gold solder bump and an aluminum bump pad, the change in the spot size over temperatures ranging from 25°C to 50°C is only approximately 0.1%. In other words, the variation in spot size is only 0.018 μm within the temperature range. This means that the athermalization is effective and that the change in temperature has only minimal influence on the collimation. Beam divergence is insensitive to temperature.
The diffractive lens was designed according to Rossi’s theory , where the phase-matching number is M=1, the number of illustrating segments is Q=7, and the diffractive optical element has a focus position that is not very sensitive on the surface-relief profile . The value M essentially determines the necessary width and depth of the microlens segments. Therefore the width and the depth of the segments are important factors for diffractive lens performance. Thermal expansion thus leads to less serious fabrication errors than do errors in the width and depth of segments; for example, errors in the depth and width of the diffractive lens relief and the curvature of the spherical refractive lens would result in a larger focal length change than would a temperature change. To increase the performance of the integrated VCSEL and hybrid microlens, the natural solution is to improve the fabrication accuracy of FIBM. Another possible improvement is to deposit an antireflection coating on the surface of the hybrid microlens. This may prevent multiple reflections at the semiconductor–air interface and the VCSEL mirrors, thus reducing the interference effects in the imaged diffraction pattern.
In summary, the hybrid microlens integrated with a VCSEL that is presented here can be applied for collimating and beam shaping. The test results show that its performance is better than the VCSEL integrated with a pure diffractive lens, especially for athermalization and decreasing dispersion, which is important for the packaging system used in free-space optical interconnection.
This work was supported in part by the Funding for Strategic Research Program on Ultraprecision Engineering from the A*Star (Agency for Science, Technology and Research), and the Innovation in Manufacturing Systems and Technology (IMST) Program, Singapore— Massachusetts Institute of Technology (MIT) Alliance (SMA program). The authors thank Choo Jian Huei for his checking the English grammar of this paper.
References and Links
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