Optimized Design of a Pump Laser System for a Spin Exchange Relaxation Free Inertial Measurement Device.

In order to improve the precision and beam quality of a pump laser for a spin exchange relaxation free inertial measurement device, we applied one scheme to achieve the square wave modulation and power stability control of the pump laser and another one to obtain the uniform intensity distribution of the laser beam, in which the acousto-optic modulator (AOM) and proportion integration differentiation (PID) controller were used to achieve the former, and the freeform surface lens was designed and optimized to achieve the latter based on the TracePro software. In experiments, the first-order diffraction light beam coming through the AOM had a spot size of about 1.1×0.7 mm2, and a spherical vapor cell with a radius of 7 mm was placed behind the freeform surface lens. Results show that the uniformity of the reshaped intensity distribution is higher than 90% within the target area with a radius of 7 mm both in the simulation and the experiment, which ensure that the uniform laser beam covers the area of cell. On the other hand, the power stability of the pump laser is controlled to be less than 0.05%. Compared with traditional methods, the complicated calculation process in optical design is better solved, and a higher uniformity with slight energy loss is achieved.

Prediction of lifetime by lumen degradation and color shift for LED lamps, in a non-accelerated reliability test over 20,000 h.

Although the predicted lifetime of the classical 6000 h test given by Energy Star is taken as the normal lifetime of LED products in most research and applications, the aim of this study is to explore the error in lifetime prediction of LED lamps based on the 6000 h test. A non-accelerated aging test with 10 LED lamps is conducted for 20,000 h (from March 2016 to now) under room temperature, which is long enough for this kind of lamp reaching the real lifetime with the normalized luminous flux dropping to 70% naturally. At different aging periods, the correspondent lifetime of each sample is predicted by the lumen degradation, and the median lifetime τ0.5 of 10 samples is obtained by applying the Weibull distribution. Result shows that the τ0.5 of the real lifetime is 16,867 h in this work, and the aging time should be at least 9000 h to make the error in predicting the lifetime less than 3%. On the other hand, the Du'v' values of 0.006, 0.007, and 0.008 are taken as the three thresholds for predicting the lifetime by color shift. For the case of 0.008, the calculated shape parameter of 8.4 in Weibull distribution is similar with that of the real lifetime, which means the Du'v' of 0.008 for this kind of lamp gives the same failure mechanism as that of lumen degradation of 70%.

Lumen degradation analysis of LED lamps based on the subsystem isolation method.

The lumen degradation of LED lamps undergoing an accelerated aging test is investigated. The entire LED lamp is divided into three subsystems, namely, driver, lampshade, and LED light source. The parameters of output power [Watts (W)], transmittance (%), and lumen flux (lm) are adopted in the analysis of the degradation of the driver, lampshade, and LED light source, respectively. Two groups of LED lamps are aged under the ambient temperatures of 25°C and 85°C, respectively, with the aging time of 2000 h. The lumen degradation of the lamps is from 3.8% to 4.9% for the group under a temperature of 25°C and from 10.6% to 12.7% for the group under a temperature of 85°C. The LED light source is the most aggressive part of the three subsystems, which accounts for 70.5% of the lumen degradation of the LED lamp on average. The lampshade is the second degradation source, which causes 21.5% of the total amount on average. The driver is the third degradation source, which causes 6.5% under 25°C and 2.8% under 85°C of the total amount on average.

Disparity between online and offline tests in accelerated aging tests of LED lamps under electric stress.

The accelerated aging tests under electric stress for one type of LED lamp are conducted, and the differences between online and offline tests of the degradation of luminous flux are studied in this paper. The transformation of the two test modes is achieved with an adjustable AC voltage stabilized power source. Experimental results show that the exponential fitting of the luminous flux degradation in online tests possesses a higher fitting degree for most lamps, and the degradation rate of the luminous flux by online tests is always lower than that by offline tests. Bayes estimation and Weibull distribution are used to calculate the failure probabilities under the accelerated voltages, and then the reliability of the lamps under rated voltage of 220 V is estimated by use of the inverse power law model. Results show that the relative error of the lifetime estimation by offline tests increases as the failure probability decreases, and it cannot be neglected when the failure probability is less than 1%. The relative errors of lifetime estimation are 7.9%, 5.8%, 4.2%, and 3.5%, at the failure probabilities of 0.1%, 1%, 5%, and 10%, respectively.

Analysis of junction temperature and modification of luminous flux degradation for white LEDs in a thermal accelerated reliability test.

An accelerated aging test is the main method in evaluation of the reliability of light-emitting diodes (LEDs), and the first goal of this study is to investigate how the junction temperature (Tj) of the LED varies during accelerated aging. The Tj measured by the forward voltage method shows an upward trend over the aging time, which gives a variation about 6°C-8°C after 3,000 h of aging under an ambient temperature of 80°C. The second goal is to investigate how the variation of Tj affects the lifetime estimation. It is verified that at a certain aging stage, as Tj increases, the normalized luminous flux linearly decreases with variation rate of microns (µ) (1/°C). Then, we propose a method to modify the luminous flux degradation with the Tj and µ to meet the requirements of a constant degradation rate in the data fitting. The experimental results show that with the proposed method, the accelerated lifetimes of samples are bigger than that of the current method with increment values from 8.8% to 21.4% in this research.

Comparison of online and offline tests in LED accelerated reliability tests under temperature stress.

Accelerated aging tests are the main method used in the evaluation of LED reliability, and can be performed in either online or offline modes. The goal of this study is to provide the difference between the two test modes. In the experiments, the sample is attached to different heat sinks to acquire the optical parameters under different junction temperatures of LEDs. By measuring the junction temperature in the aging process (Tj1), and the junction temperature in the testing process (Tj2), we achieve consistency with an online test of Tj1 and Tj2 and a difference with an offline test of Tj1 and Tj2. Experimental results show that the degradation rate of the luminous flux rises as Tj2 increases, which yields a difference of projected life L(70%) of 8% to 13%. For color shifts over 5000 h of aging, the online test shows a larger variation of the distance from the Planckian locus, about 40% to 50% more than the normal test at an ambient temperature of 25°C.