Optical Waveguide Component

A Novel Alignment Technique for Active Optical Waveguide Components

Abstract: In this paper a novel technique for the alignment of different active optical waveguide components is reported. This technique has successfully been implemented for the characterization of waveguide a photodetector with an active layer thickness of 0.15 mm. This technique is easy to use and is very accurate and reliable.

I. INTRODUCTION

It is well established that monolithic optoelectronic integrated circuits (OEICs), which incorporate electronic circuits and optical components (free-space or connected via optical interconnects or waveguides) on the same substrate, will play an important role in future data transmitting and processing systems. Due to their importance, OEICs are investigated extensively in order to take full advantage of optical means of data transmission and processing. Although monolithic integration is an ideal approach for the integration of several components on a single substrate; however, the integration of several components on a single substrate raises the question of routing the signals, because light generated in the one part of the device will be strongly absorbed in other parts of the device. Hence, mainly due to this problem, this approach is still a technological challenge. Large efforts are needed to solve the problems associated with monolithic approach step-by-step. In order to have a better understanding of light propagation in active waveguide components, a hybrid approach has been used as an intermediate step occasionally. The major advantage of a hybrid approach is that the performance of each component can be optimized according to a particular application. However, hybrid integration suffers severely from the poor coupling of light from a source to a component. To overcome this problem, several passive and active alignment techniques have been suggested [1-4], but they all need sophisticated fabrication techniques for implementation. Furthermore, they are very much limited to a particular device geometry. Normally in a typical laboratory environment, butt-coupling alignment technique [5] or a free-space optical lens system with or without a tapered fiber is used to couple the light input to a component. In a butt-coupling technique, components have to be very close to each other; otherwise, this technique suffers from beam divergence. If two components are placed very close to each other, temperature effects, which cause expansion of material, cannot be ignored. This could damage the facets of the device being used. Furthermore, it becomes very difficult to isolate the laser source from reflection from other components. On the other hand, the tapered fiber technique also suffers from relatively high coupling losses of optical radiation. Nevertheless, this technique has a reasonably large working distance and offers safety to the optical components from accidental facet damage. In both techniques, the alignment of different components is difficult, time consuming and may not be reliable. Here a reliable and easy to use technique for the alignment of active optical components is demonstrated by making use of optical radiation detection properties of a semiconductor laser diode. This technique is basically intended for a typical optoelectronic laboratory set-up where the different materials and components are needed to be characterize for future monolithic OEICs.

II. EXPERIMENTAL PROCEDURE

To demonstrate the validity of the proposed technique, two identical 2.5 m wide, 250 m long rib waveguide stripe geometry GaAs laser diodes, UB259 and UB262 were used.. The active layer thickness was 0.15mm both the devices. UB262 was used as the source laser and UB259 was used as the active waveguide optical detector (AWOD) [6]. A single layer antireflection (AR) mcoating was applied to both facets of UB259 to reduce the facet reflectance in order to allow the maximum input power to couple in to the AWOD. Both devices were mounted on two separate copper heat sinks blocks. The temperature of both devices was controlled independently using thermoelectric Peltier cooling devices and a three channel temperature controller. The laser source was biased with 300ns long pulses with a repetition frequency of 10 kHz by using a HP8082A pulse generator to avoid overheating in order to achieve more reliable results whereas AWOD was used under no bias conditions. This proportion between the pulse width and pulse repetition frequency was sufficient to isolate the transient temperature effects caused by one pulse from other pulses. Along with an optical lens system, a pre-calibrated large area Si detector (LAD) was used to measure light output and a infrared camera was used to observe near fields patterns of LD and AWOD in place of LAD as and when required. All the results were plotted using a Tektronix sample & hold oscilloscope and an X-Y plotter. Figure 1 shows the schematic of experimental set-up used to implement this alignment technique.

First of all AWOD being and the laser source were aligned with the help of IR camera in such a way that it was obstructing the optical path between the source laser and IR camera. By manipulating the position of the source laser, lens-1, lens-2 and AWOD and observing the near field beam profile of the source laser by using the camera, a slight response of the AWOD was observed which was maximized by repositioning the input beam on to the input facet of AWOD. This maximum output of AWOD was representing the maximum alignment between the two active components. In order to check the reliability of alignment between the source laser and AWOD, UB262 was used as an AWOD (under no bias condition) and the UB259 as a source. The response of UB262 as an AWOD was found maximum at a point where the response of UB259 (used as AWOD) was maximum (in this case, no optical isolator was used).

Lens 2

X20

NA .45

Camera

Sample &

Hold Oscilloscope

Plotter

AWOD

Lens 1

X20

NA .45

Optical

Isolator

Fig. 1. Schematical representation of experimental set-up used for the implementation of the proposed alignment technique.

III. RESULT AND DISCUSSION

After achieving the maximum alignment, the response of an AWOD was analyzed by measuring the I-L characteristic of the source laser using UB259 as an AWOD. Also the I-L characteristic of the source laser was measured by placing LAD just after lens-2 (in place of AWOD). The results of both measurements are given in fig. 2. It can be seen from fig. 2 that the response of a pre-calibrated Si LAD and the GaAs AWOD are similar to each others except the sensitivity. This indicates that there was a perfect alignment between the source laser and AWOD. To estimate the sensitivity of the GaAs AWOD, the output of LAD is plotted against the photocurrent through AWOD. Figure 3 shows the relationship between the input optical power to AWOD and the photocurrent through AWOD. From fig. 3, the sensitivity of the AWOD was calculated to be 0.22 A/W. This value is well within the reported value of refs. 7 and 8. This shows the validity and reliability of the proposed technique. However, this technique was found to be very sensitive to temperature variations because any change in temperature was changing the length of AWOD and hence causing a shift in the initial position of AWOD facet. Change in length with respect to temperature can be calculated as [9];

(1)

where α is expansion coefficient, L is the length of the device and dT is the change in temperature with respect to reference temperature.

Change in length due to temperature variations is usually ignored due fact that the change in cavity length is expected to be relatively small for GaAs heterojunction structure due to small expansion coefficient. But it was observed that a slight change in operating temperature was degrading the coupling efficiency drastically as shown in fig. 4. This curve was measured in the following manner: First, the system was aligned by keeping the source laser and AWOD temperature at 20^oC. After achieving the maximum alignment, the temperature of AWOD was brought down to 15^oC. The response of AWOD was measured at this temperature (point 'A' in fig. 4). The temperature of AWOD was again increased to 20^oC and point 'B' is representing the output of AWOD at that instance. The temperature of AWOD was further increased to 26^oC and the response of AWOD was again measured low (point 'C'). To check the effect of temperature variation on the alignment, the temperature of AWOD was brought back to original value of 20^oC (point 'D') and nearly the same output from AWOD was achieved as at point 'B'. This shows the reliability of the technique. Temperature dependence of alignment can be explained by considering the temperature dependence of cavity length. At point A, due to decrease in the temperature, the effective length of laser cavity was decreased and the incoming light beam was focused in front of the input facet of AWOD. At point C it was focused beyond the input facet of UB259 due to increases in effective cavity length. Whereas, at point B and D, the system was aligned, and the input beam was focused exactly at the input facet of AWOD.

For L = 250 m, α = 5.85 x 10^-6/K [10,11], and dT = 5^oK with respect to reference temperature of 20^oC , dL is calculated as around 7.3 nm. It is clear from the above arguments that this technique was able to pick-up a slight change in the length of the device. Hence this technique can also be used to monitor change in length of any active device occurring due to different effects. This technique was able to pick-up these small changes in length of the device

V. CONCLUSION

In this paper, an active alignment technique for active waveguide components for future free-space optoelectronic integrated circuit is demonstrated and implemented successfully.

The accuracy of this technique is expected to be in sub-micron region due to the fact that the thickness of the active regions of both devices was 0.15 mm and slight misalignment would definitely have reduced the amount of detected power. Although this technique has been implemented on GaAs based devices, yet this technique can be used for the alignment of any active device.

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