Accelerated Aging

The operation of a sample population of the lasers for an extended period of time under harsher conditions then those specified. These conditions are believed to accelerate degradation processes. Care must be taken not to introduce degradation processes that would not be relevant under operation within specified conditions

Aging

The operation of a sample population of the lasers for an extended period of time under specified conditions. BLI performs aging between 500 hours and 10,000 hours.

Beam Divergence (q)

The emission intensity of the laser beam radiation measured as a function of the angle. The value is normally measured from the full width at half maximum (FWHM) of the emission intensity. Such measurement is giving in two different directions: one perpendicular to the p-n junction and the other parallel to the p-n junction.

Beam Steering

The center of the far field distribution can change as a function of operating conditions such as time, temperature, and operating current. The far field in the perpendicular direction is mainly determined by the epitaxial structure. Temperature and injected current can change the index of refraction and the optical field. These changes can shift the optical beam.

At BLI the maximum shift is +/- 2 degrees. The far field in the parallel direction is more complex. BLI laser is a multimode device. The far field is a result of interference between many modes. The far field is generally a two lobed pattern. However, depending on operating conditions, energy is shifted from one lobe to the other. For 100 micron aperture devices, the maximum shift is +/- 3 degrees.

Burn-in

The initial operation of the laser diode at the manufacturing facility to eliminate infant mortality failures, devices prone to catastrophic failure, and devices that have degradation rates exceeding customer specifications. The effectiveness of the burn-in depends on the operational conditions during burn-in, the burn-in time, and the acceptable changes in device performance during burn-in.

BLI varies the burn-in to provide the desired reliability at the least expense to the customer. The effectiveness of the burn-in is verified with sample aging.

Catastrophic Failure

A failure in which the device characteristics change suddenly, generally in less than a second.

Catastrophic Optical Damage (COD)

Catastrophic optical damage occurs when the semiconductor in the emission region is melted and recrystallized. The recrystallized material is highly defective (analogous to scar tissue). This defective material in the emission region may not be observable using optical microscopy. The COD limit of a laser diode can only be determined by increasing the output power until the laser fails catastrophically. The COD limit is the output power when the laser fails divided by the emission area.

The practice in the industry is that the COD limit should exceed the output power density during operation by a factor of two to three. The COD limit for most BLI high power laser diodes in the 800 nm to 980 nm spectral region is approximately two megawatts per square centimeter.

Dark-Line Defect (DLD)/ Dark-Spot Defect (DSD)

Failures due to DLDs and DSDs show regions of lower luminescence efficiency. DLDs and DSDs are linear and circular in shape, respectively. Detailed electron microscopy shows these defects to be due to non-radiative recombination at dislocation networks.

Emitter Size (d)

The area from which the light is generated in the front facet of the laser diode is called the emission aperture. In BLI laser diodes, the emission aperture is located approximately three microns from the p side of the laser diode. The emission aperture is approximately one micron in the dimension perpendicular to the p-n junction. The width of the emission aperture is processes dependent and can be customized. Standard widths of the emission aperture for BLI laser diodes are 100, and 200 microns.

Failure

Change in performance characteristics of a device, so that it no longer meets customer specifications.

Failure mode analysis (FMA)

The examination ("autopsy)" of a failed laser diode by a failure mode analyst ("medical examiner") to determine the cause of device failure ("death"). It is important to realize that a high power laser diode is approximately 100 microns by 500 microns by 1000 microns in size. Not surprisingly, the cause of laser failure is often undetermined

Full Width Half Maximum (FWHM)

The measure of the extent of a function.  Given by the difference between the two extreme values of the independent variable at which the dependant variable is equal to half of its maximum value.  The term duration is preferred over width when the independent variable is time.  Commonly applied to the duration of pulse waveforms, the spectral extent of emission or absorption lines, and the angular or spatial extent of radiation patterns.

Gradual Failure

A failure in which the device characteristics change slowly, anywhere from hours to years.

Infant Mortality

Failures that occur within a short time of initial operation, generally in the first few hours.

Mean-time-to-failure (MTTF)

The average of the TTF of each laser in a population. It should be noted that the acronym "MTTF" as used in the literature may also refer to median-time-to-failure.

Median-time-to-failure (MTTF)

The projected time when half of the lasers in a population fail. It should be noted that the acronym "MTTF" as used in the literature may also refer to mean-time-to-failure.

Operating Current (Iop)

The forward biased current through the diode necessary to produce the output power (P). Iop is determined by:

• the device structure of the diode
• the material composition of the diode
• the front and back facet reflectivity

Iop depends empirically on exp(T/To) where T is the absolute temperature of the active layer. To is a constant characteristic of a device. For BLI's quantum well laser diodes that operate in the 800 nm to 850 nm spectral range, To is approximately 150 degrees.

Operating Temperature

The heat sink temperature at which the diode laser characteristics were measured.   

Operating Voltage (Vop)

The forward voltage across the diode necessary to produce the output power (P). Vop has two components, the voltage across the p-n junction and the voltage due to device resistance. Vop is determined by:

• device structure
• material composition
• device fabrication and packaging

Optical Output Power (P)

The maximum rated optical output power from the front of the laser diode under continuous wave (CW) operation. P is determined by:

• optical power density (power per unit of emission area);
• device structure;
• material composition;
• device fabrication and packaging.

Parallel Beam Divergence (q)

See Beam Divergence

Perpendicular Beam Divergence (q^)

See Beam Divergence

Polarization Ratio

Light from laser diode is linearly polarized with the electric field parallel to the p-n junction. Polarization ratio is the ratio of intensity of light polarized parallel to the p-n junction to intensity of light polarized perpendicular to the p-n junction.

BLI laser diodes have polarization ratios in excess of fifty (50).

Slope Efficiency (h)

Increase in the output power (P) divided by the increase in operating current (Iop). h is determined by:

• device structure
• material composition
• device fabrication and packaging
• front and back facet reflectivity

Threshold Current (Ith)

The forward bias current at which the diode starts to emit stimulated emission (thus becoming a laser, or lasing). Ith is determined by:

• device structure
• material composition
• device fabrication and packaging
• front and back facet reflectivity

Time-to-failure (TTF)

This term applies only to gradual failures. In general, the characteristics (e.g. optical power, threshold current, operating current) of the device under consideration is still within customer specification but is changing very slowly. This change is extrapolated in a linear fashion to the time when the characteristic is out of specification.

Visible optical damage (VOD)

Visible optical damage is a damage, which occurs due to photo-induced breakdown of trace (as low as parts per billion) organic chemicals leaving on the surface of the emission region a highly absorbing film that is rich in carbon. This film is clearly observable using either optical or electron microscopy and is localized over the emission region. VOD failures can often be mistaken for COD failures. The emission region of a VOD failure is discernible by optical microscopy. The semiconductor in the emission region of a VOD failure in the initial phase of degradation is undamaged. VOD can be caused by particulates deposited on the surface of the emission region. Even a particle that has a low absorption heats up rapidly when exposed to a power density of megawatts per square centimeter

Wavelength (l)

The wavelength of a laser diode is determined from its emission spectrum dependant upon the following variables:

• device structure
• material composition
• device fabrication and packaging
• front and back facet reflectivity

It is temperature dependent and approximately increases by three (3) Angstroms for each Centigrade increase in diode temperature.  The wavelength tolerance of a diode laser is the acceptable amount of variation in the emission spectrum due to wafer material and processing.