How to evaluate the lifespan of diodes in laser therapy equipment?

1, The core influencing factors of diode lifespan
The lifespan of laser diodes is constrained by multiple factors, among which temperature, current, and optical power are the three key variables:

temperature effect
For every 10 ℃ increase in diode junction temperature, the lifespan is reduced by 50% -70%. For example, for a GaAlAs laser diode with a wavelength of 850nm, the threshold current increases by about 1% for every 1 ℃ increase in temperature; The threshold current of a 1300nm wavelength InGaAs laser diode increases by about 2% for every 1 ℃ temperature rise. High temperature can accelerate cavity surface oxidation, dislocation growth, and metal diffusion, leading to electrode degradation or bonding failure.
Current stress
When the driving current exceeds 80% of the rated value, the diode enters a high stress state, non radiative recombination increases, and the luminous efficiency decreases. For example, a certain model of laser diode accelerates aging at 70 ℃ and 1.2 times the rated current, and the calculated mean time between failures (MTTF) exceeds 100000 hours. However, in actual use, if the current fluctuates frequently, the lifespan may be significantly shortened.
optical power density
High power density can exacerbate cavity surface optical damage (COD), especially in pulsed operating mode, where the instantaneous peak power may exceed the cavity surface damage threshold, leading to catastrophic failure. For example, a high-power laser diode has an average lifespan of 2.19 × 10 ⁹ pulses at a duty cycle of 10%, a current of 90A, and a water temperature of 20 ℃; When the water temperature rises to 35 ℃, the lifespan decreases to 1.65 × 10 ⁹ pulses.
2, Standardized testing methods for life assessment
To shorten the evaluation cycle, the industry generally adopts Accelerated Aging Test (ALT), which simulates long-term use scenarios by increasing temperature or current, and combines statistical models to calculate the actual lifespan:

Accelerated Aging Test Mode
Constant power mode (APC): maintains the output optical power constant through a feedback circuit, simulating the actual working state. For example, a certain testing system uses external photodetectors or internal monitoring diodes to monitor power in real time. When the output power decreases by 20% or the driving current increases by 20%, the lifespan is determined to be terminated.
Constant current mode (ACC): Keep the driving current constant and monitor the changes in optical power over time. This method is suitable for studying degradation mechanisms, but it differs significantly from actual working conditions.
Key testing parameters
Threshold current (Ith): reflects the growth of defects in the active region. During the aging process, Ith increases logarithmically with time. When Ith reaches 1.5 times the initial value, it is generally considered that the diode has failed.
Slope efficiency (η): characterizes the photoelectric conversion efficiency. A 30% decrease in η or a 50% decrease in output power can be used as a criterion for end-of-life.
Forward voltage (Vf): reflects the change in electrode contact resistance. An abnormal increase in Vf may indicate bonding degradation or metal diffusion.
Statistical Models and Extrapolation of Lifespan
Based on the Arrhenius equation, extrapolate the room temperature lifespan through high-temperature acceleration test data. For example, the lifetime of a certain laser diode is 2300 hours at 70 ℃, and the lifetime at room temperature (25 ℃) can be extrapolated to over 100000 hours by calculating the activation energy (Ea=0.7eV). In addition, the log normal distribution model can be used to analyze the median life and failure rate distribution.
3, Failure Mode Analysis and Life Optimization Strategy
The failure of laser diodes can be divided into three categories, and targeted optimization measures need to be taken:

Early Failure
Caused by manufacturing defects (such as dislocations, cavity surface contamination) or packaging issues (such as heat sink virtual soldering), typically occurring within 50-100 hours of initial operation. The solution includes:
Strict screening: Early failure devices are removed through high-temperature aging testing.
Optimized packaging: Adopting eutectic welding, low thermal resistance heat sink, and airtight packaging to reduce thermal stress.
Accidental failure
Caused by external factors such as electrostatic discharge (ESD), electrical surges, or mechanical vibrations. The protective measures include:
ESD protection: Integrate TVS diodes in the driver circuit to limit voltage spikes.
Surge suppression: Using a soft start circuit to avoid sudden changes in current.
Wear and tear failure
The main cause of end-of-life is material degradation, such as cavity surface oxidation and metal diffusion. Optimization directions include:
Material improvement: Adopting non absorbing cavity surface (NAB) technology to reduce thermal damage caused by light absorption.
Heat dissipation design: Use microchannel coolers or semiconductor coolers (TECs) to control the junction temperature within a safe range.
Driving strategy: Using pulse width modulation (PWM) or dynamic power control to reduce the average optical power density.
4, Industry application cases and data support
Medical laser equipment case
A certain model of diode pumped solid-state laser (DPL) is used for dermatological treatment, and its lifespan is defined as ending when the output power is below 70% of the rated value. By optimizing the polishing process of the frequency doubling crystal (KTP) and controlling the power density inside the cavity, the lifespan of the laser has been extended from 5000 hours to over 10000 hours.
High power laser diode data
A quasi continuous wave (QCW) laser diode has an output power of 91W, a slope efficiency of 1.16W/A, and an average lifespan of 2.19 × 10 ⁹ pulses at room temperature and a duty cycle of 10%. By improving the multi-layer soldering packaging process, the environmental temperature tolerance has been increased from 20 ℃ to 35 ℃, and the lifespan degradation rate has been reduced by 25%.