Why Beam Profiling at 1550 nm Requires InGaAs Cameras

Gary Wagner, General Manager (U.S.), Ophir Photonics

The initial test was conducted with a Spiricon SP620-1550 USB camera consisting of a standard silicon array of 1600x1200 pixels on 4.4um pitch, producing a minimum dynamic range of ~30dB. The silicon array is coated with a thin layer of proprietary phosphor making the pixels sensitive to detection and imaging of light sources in the NIR. This approach is quite effective when imaging a light source directly onto the array, especially with 1-2mW of power. However, due to the extremely low sensitivity of this material, it is nearly impossible to image a reflected beam off an image plane as was required in this application.

The expected absorption curve for the Spiricon Phosphor CCD cameras is shown in Figure 1.

The results of this initial test were totally unsatisfactory. No effective image of the beam could be detector or profiled. The low absorption of the phosphor coated CCD prevented any useful data from being gathered from this approach.

olution: The Spiricon Xeva XC130 InGaAs camera was then installed onto the optical table. The resolution of this camera is 320 x 256 with a 30um pitch producing low gain of 68dB. This degree of sensitivity allows the camera to detect very low power images from either a direct source or a beam reflected off an image plane as was needed in this application.

The InGaAs detector absorbs well between 900 and 1700 nm.

Due to the significant increase in sensitivity of the XC130 camera, images of the beam from different reflected arrangements were now possible with great resolution. The fixture set up showing the XC130 InGaAs camera with a reducing lens attached is in Figure 2.

In this optical arrangement, the beam source was toward the left of the camera, reflected off an image surface to the right of the camera, back onto a reflective surface into the lens of the XC130 camera. With signal loss at each beam transfer, the XC130 camera using Spiricon BeamGage Professional beam diagnostic software was more than sufficient to image and profile the resulting image.

The typical progressive images of this 1550nm source from different optical arrangements provided the following results.

Due to the changes in power density of the images depending upon power settings and the type of image plane, the XC130, using the Frame Summing feature when needed, was easily able to bring out the full beam pattern, size and depth to a point where accurate dimensional data and intensity values could be obtained and documented for the analysis of the project.