Why Beam Profiling at 1550nm Requires InGaAs Cameras

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

A recent application called for the beam analysis of a 1550nm laser source with a challenging optical arrangement. Signal loss occurred at each beam transfer across multiple reflective surfaces. A more sensitive camera with Frame Summing was required to bring out the full beam pattern, size, and depth.
The 1550nm laser source was measured with an Ophir Laser Power Sensor, PD300- IR, at 1.5mW, with a beam diameter of 3.5mm. The lab resources were limited to using a phosphor coated CCD camera that is sensitive to the wavelengths between 1140nm and 1610nm. The requirement of the analysis necessitated that the beam be reflected off different surfaces to document the changes of the beam image relative to absorption and reflection.

Phosphor Coated CCD Camera Absorption

  Figure 1. Phosphor Coated CCD Camera Absorption.
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.
Solution: 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.
Fixture Setup with XC130 Camera

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.

Figure 2. Optical Setup with XC130 Laser Beam Profiler  
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.
Beam Profile of 1550nm Source using BeamGage
Figure 3. Beam Profile of 1550nm Source using BeamGage.
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.

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