Noise Measurement Virtual System Of Semiconductor Laser

The opto-electronic technique is the main part of today's advancedtechnology fields. It becomes more and more important in Country Economyand the construction of natural defense. With the rapid development ofopto-electronic technique, semiconductor lasers (LDs) are widely used inoptical fiber communication, optical sensor, information memory, medicaltreatment and pumping solid lasers. With the expansion of the excitonicwavelength, Spectral characteristic's improvement, the reducing of thresholdcurrent, the improvement of quantum transfer efficiency, the enhancement ofoutput light power, the improvement of reliability etc, the superiority of theLD is more obvious. As a kind of important light source, the reliability ofsemiconductor lasers is imperative in all the applications. The study on thereliability of LDs and its regularity is very important to improve their specificproperty and use them rightly. At present, the usual method of screen iselectric aging in which all devices are aged with a constant power or a constantcurrent under high temperature and then the early failed devices are selected.In the aging, the reliable devices are also subject to screening, and the lifetimeof devices is affected because of the hot and electric hurt. It can cause batchesof devices damage when some accidence happens, for instance, the out ofcontrol system or power cut. On the other hand, electric aging is not sensitivefor latent defects;for example, some devices after electric aging screening areusually found fail rapidly. Thus, the urgent problems are how to manufacturethe high reliable devices, how to analyses the factors, which affect the devicereliability, and how to select the unqualified devices with a fast,nondestructive and conveniently method. When LDs work, p-n junctionvoltage of it will tend to stochastic fluctuation, named electronic noise.Electric noise indicates the defect of material and device;especially it is verysensitive with latent bug, which will affect the reliability of the device. So itshows great potential in quality token and reliability estimation of the device.So it is an important work to develop a noise test system of LDs, which have alow price and have entire test functions.This paper gives the introduction of LDs noise characteristic first. Thenoise includes light noise and electric noise ,light noise includes phase noiseand intensity noise, electric noise includes thermal noise ,shot noise,1/fnoise, g-r noise and so on. This paper gives a detail discussion of electricnoise which includes the feature,generate mechanism,equivalent circuit andapplication. Then the paper introduces the noise measurement of LDs whichincludes direct measurement,commutative spectrum measurement,digitalmeasurement based on PC,multi-channel measurement and interferencemeasurement. Now we often use the first measurement, that is pre-amplifier +spectrum analyzer. In this measurement, we amplifies the measured signal,and the put it to spectrum analyzer to do some analysis. But usually thespectrum analyzer has no other than two or four ports, the same that it candetect at most two or four devices at the same time. However practically weare in need of detecting more than ten devices at the same time, the spectrumanalyzer is obviously insufficient. With the developing of the PC technique, allkinds of data acquisition boards or cards are created and more and moreperfect;meanwhile the virtual instrument technique is also developing in nearyears. These make us think that, if we make use of the software functions, usethe computer instead of the spectrum analyzer and select a multi-channel dataacquisition card, we can solve the problems and lower system costs. In fact,these aren't too hard to do. Meanwhile, the cost of spectrum analyzer is alwaystoo high, if we make use of the virtual technology, it will reduce the costgreatly. So my main work during the period of doing my paper is to give adetailed design of a LD noise virtual test system.This system mainly includes hardware part and the software part. Themain work in the hardware part is: put the tested signal, which generates fromthe test box into a low noise pre-amplifier (Model 5184 made by EG&Gcompany) to be amplified. The amplify signal with our design: an amplifierdesigned with MAX412, this process is to make the range of the signal to thedemand of data acquisition card. Data acquisition system is the main port ofour design, it has many features: high speed sampling,multi-burst,save dataand A/D transfer,D/A transfer. We use PCI-6014 data acquisition card madeby NI. During the whole part of hardware design, because of the tested signalis electric noise which between several to ten nV, and it is very faintness, howto separate other noises from the test system is very important. We use manymethods: choose the proper devices, give a proper layout, do mask etc.Software part is the core part of our test system, that is because my paper'sinnovative part is design a virtual spectrum analyzer based on Lab VIEW toreplace the existing instrument to realize "Software is instrument". The virtualinstrument can display waveform in time domain and frequency domain,analysis spectrum, analysis power spectrum, analysis power density spectrum,cross-power spectrum analysis, frequency corresponding analysis etc. At thesame time, the virtual instrument has the function include filter,window,average to signals. We use Lab VIEW 7.0 which developed by NationalInstrument for virtual instrument's design, we can write a program by graphsand it contains many functions which make our work more quickly andconvenient. The surface of the virtual instrument is beautiful. like the realinstrument. The virtual instrument is can be modified easily.The key of the whole system is the preamplifier, whose noise voltagedecides if the system can acquire the valid datum and analyze correctly. Atpresent, we are researching to design a preamplifier whose noise voltage islower. At the same time we adopt a more valid shielding method to reduce thenoise of the whole system .The system is designed for the noise measuring ofsemiconductor lasers, but it also has the feature of the traditional and availableinstruments. If the hardware part is modified simply, the system can measureany electric signals or the other signals. So the system put forward by thepaper has extensively applying value.