Application of Digital SourceMeter B2900A in Semiconductor Laser Testing

Semiconductor lasers are core components in the fields of optical fiber communication, laser display, gas detection, etc., and have attracted extensive attention from scientific and technological personnel around the world. In the production and development of semiconductor lasers, the measurement of the optoelectronic characteristics of lasers is particularly important, and is a key link in controlling the stability of the laser preparation process and the reliability of laser performance.

Semiconductor lasers are semiconductor photoelectric conversion devices. As shown in Figure 1, semiconductor lasers are composed of multiple layers of materials. From bottom to top, they include back electrodes, substrates, lower light confinement layers, lower waveguide layers, active layers, upper waveguide layers, upper confinement layers, and upper electrodes. Different layers are composed of different epitaxial materials. Such a layered structure is to achieve 1) injection and recombination of carriers (electrons and holes) for luminescence, and 2) lateral confinement of photons to form an optical waveguide. The epitaxial layered structure must undergo an etching process to form a ridge waveguide, and a contact electrode is prepared on the ridge waveguide. The purpose of such a ridge waveguide is to 1) limit the lateral diffusion of current and 2) form a lateral waveguide for photons. The prepared wafer is subjected to processes such as cleavage, coating, welding, and wire bonding to obtain the laser to be measured, as shown in Figure 2. When current is injected into the laser electrode, electrons and holes on both sides of the laser PN junction flow into the active area in large quantities, where electron-hole pairs recombine to generate a large number of photons. The photons propagate along the axis under the action of the waveguide. At the end face of the laser, the reflected light forms the lasing condition, and the transmitted light is the laser output by the laser. The working characteristics of the laser are mainly reflected in 1) PN junction characteristics, series resistance, 2) laser lasing threshold, and laser slope efficiency. These characteristics determine the properties of the laser such as optical power PO, power conversion efficiency ηP, and working life. PIV measurement methods are often used to obtain these important parameters in production and scientific research.

The PIV characteristics of semiconductor lasers are the response characteristics of the laser's light output power P and the voltage V between the laser's two poles when the laser is injected with current I. Unlike ordinary two-terminal devices such as PN junction diodes, the PIV test of semiconductor lasers not only measures the voltage-current (VI) characteristics of the laser, but more importantly, it also completes the test of the laser's power-current (PI) characteristics. Therefore, the semiconductor laser PIV test system includes a precision current source, a voltmeter, a power meter, and a software part responsible for control, inter-instrument communication, data acquisition, and processing. The traditional PIV test system consists of a discrete current source (with an integrated voltmeter), an ammeter, and control software. The current source is used to provide the laser injection current and measure the voltage. The ammeter is connected to the integrating sphere to measure the laser power. The current source and ammeter are connected to the software through the GPIB port to complete tasks such as measurement triggering and data transmission.

There are many problems with complex PIV test systems, including 1) low measurement speed and a small number of points measured per unit time, which increases measurement time; 2) poor system stability, data transmission blockages between instruments connected by the GPIB port, and the system is prone to paralysis; 3) poor system controllability, and the measurement speed, number of scanning points, and scanning mode cannot be adjusted according to measurement requirements. In the process of a large number of semiconductor laser optoelectronic tests, traditional PIV test systems have problems with low efficiency, reliability, and poor controllability.


B2900A Integrated PIV Test System

The B2900A integrated PIV test system (referred to as the integrated PIV system) was designed and built by this experimental group. The PIV test was performed using a combination system of a precision source measurement unit SMU and an integrating sphere detector. The system has a simple structure, high precision, good reliability, and fast speed. While improving production efficiency, it also increases test accuracy and reliability, reducing the cost of a large number of tests. The integrated PIV system is mainly composed of the B2900A dual-channel precision source measurement unit SMU, an integrating sphere, a fixture, and software. One channel CH1 of the B2900A is used as the current source of the laser and measures the voltage V of the laser at the same time. The laser output light of the LD is coupled into the integrating sphere, and is converted into photocurrent by the integrating sphere detector and enters another channel CH2 of the B2900A. The photocurrent measured by B2900A CH2 is multiplied by the power-to-current conversion coefficient of the integrating sphere detector to obtain the output light power of the laser. The B2900A is connected to the PIV measurement software via a USB port to complete measurement control and data acquisition. Figure 3 is the experimental connection diagram. During measurement, the test parameters are set through the PIV test software, CH1 and CH2 are triggered internally to perform PIV measurement at the same time, and the data is returned to the software in real time through the USB port. The dynamic characteristics of the laser can be observed in real time through the software interface. At the same time, the characteristic parameters of each laser can be quickly analyzed based on the software measurement data, including forward voltage drop VF, optical power PO, threshold current Ith, inflection point and other parameters Ikink.

The integrated PIV system has shown its superior performance of easy operation, high precision, high efficiency and high reliability in the process of conducting a large number of laser PIV tests. All test parameters of the laser can be easily set on the software interface, including current source voltage source switching, DC pulse switching, current range, limit voltage, scanning mode, number of scanning points, scanning time and speed, etc. This makes parameter adjustment between different measurement conditions convenient and fast. Secondly, the current measurement accuracy of the B2900A precision source measurement unit can reach 0.1nA. When measuring high-resistance semi-insulating structures (107~8Ω), the resistance value of the high-resistance structure can be accurately measured at a small driving voltage (<1V). At the same time, the power of the laser is measured synchronously with the voltage and current, without the need for external coordinated triggering. During scanning measurement, more than 400 points can be measured within 8 seconds.

The scalability of the integrated PIV system is also an outstanding advantage in its application in the field of semiconductor laser research and development. The hardware part of the integrated PIV system includes the B2900A precision measurement unit and the measurement platform, which are connected to the computer via ports such as GPIB or USB. Therefore, the hardware part of the integrated PIV system can cooperate with the test software program written by the user through software platforms such as National Instrument to complete the more complex measurement tasks required by the user.

In the production and development of semiconductor lasers, a large number of PIV tests are required for semiconductor laser chips. Compared with the traditional discrete and complex PIV test system, the PIV test system integrated with the B2900A precision measurement unit has the advantages of simple system structure, convenient operation, high accuracy, strong reliability, and rapid measurement. It greatly reduces the operating cost and time cost of PIV performance measurement of laser chips, and increases the flexibility of PIV measurement. The B2900A precision measurement unit will be widely used in the fields of semiconductor laser production and scientific research.