Laser Diodes: Specification Guidelines
Intense Ltd.While some design engineers know exactly what they need from their laser diode, others may choose a partner to navigate what can be a complex development process. Laser diode manufacturers know which specifications have the greatest impact on costs and can help you find the most cost-effective solutions for your application.
Figure 1. High-power laser diodes.
Manufacturing can explain why some wavelengths are less expensive than others, when one package is more appropriate than another, and why testing is critical before committing to a production order.
For example, the high power laser diodes shown in Figure 1 need thermally compatible heat sinks that may compromise package size. Contacting the diode manufacturer early in the design process can save time, money, and frustration.
Find a supplier with the expertise to produce the lasers. The supplier must have the flexibility and knowledge to customize a product and the capability to manufacture it in the volumes needed. Developing close, interactive communication with a supplier willing to provide one-on-one service is a must.
Figure 2 shows the 10-step process that will help you determine your laser diode needs and your supplier’s ability to meet those needs.
Figure 2. A 10-step process will help you determine your laser diode needs and your supplier’s ability to meet those needs.
The design and prototyping process can be quite involved. It may require extended and close interactions with the diode manufacturer when establishing basic design needs. Issues to discuss include performance levels, product lifetime, special environmental testing, delivery schedules, drawings, spec sheets, source and quality inspections, and budgets. These requirements are preliminary and can change as you work with the manufacturer to balance the whole package of specifications to fit the application. This balance, necessary for a cost-effective and reliable product, happens only if your supplier can describe the potential trade-offs, including the following:
Avoid open packages — Open packages are space-efficient and easily accessible, but they leave the fragile laser exposed to contamination and mechanical damage. Most suppliers will not guarantee open packages because the possibility of damage is so great. Sealed packages protect the device from mechanical damage and provide a particle-free and dry atmosphere for the laser.
Consider active temperature control — An increase of 10 °C may reduce a laser’s lifetime by 50 percent. Many sudden and longer term failures have been traced to inadequate laser cooling. Passive cooling can be appropriate for some big applications, but even then an engineer should determine the temperature under worst-case conditions and plan appropriately.
Always use laser drivers with the appropriate protection — The most common cause of failure in laser diodes is current transients. Look for well-qualified commercial laser drivers. Testing and constructing a custom driver requires expertise and specialized equipment to ensure that the driver adequately protects the laser under all conditions. Electrostatic discharge usually caused by poor handling techniques, also plays a large role in laser diode failure. Anyone working with laser diodes must be thoroughly familiar with techniques for preventing this problem.
Operate the device at rated or lower output — Standard-brightness laser diodes emit about 1 W per 100 µm of stripe length; high-brightness devices may emit twice as much or more for the same stripe length. However, increasing brightness increases cost and may affect lifetime. Many OEMs who use devices in critical applications operate them below the rated power. In lower duty cycle applications, it may be possible to exceed the rated power, but one should assess the reliability consequences beforehand.
Use standard wavelengths and loosen tolerances where possible — Most manufacturers produce devices that cover a definitive wavelength range. Within this range, some wavelengths will be more commonly available than others and are more likely to be in stock. For example, a company’s lasers might cover the range from 635 to 980 nm, with standard device wavelengths at 635, 650, 670, 785, 808, 830, 910 and 980 nm (Figure 3). If an application demands a special wavelength, it may require growing a custom wafer, which can add costs of $10,000 to $30,000.
Figure 3. High-power laser diode suppliers typically offer standard wavelengths across the spectrum.
Use standard packages — The industry has developed a number of standard packages, including C-mount, TO-3, and HHL, and each laser diode supplier has test and burn-in fixtures designed for them. Using standard packages reduces cost and delivery times because the supplier doesn’t need to pass on extra expenses of custom fixturing. Moreover, custom packages typically require three- to four-month lead times and may also require an investment in custom tooling. In large quantities, however, custom packages are not necessarily much more expensive than the standard ones. Finding a package supplier willing to do custom work is another issue.
Don’t specify characteristics that are hard to control — Some characteristics, such as tight wavelength tolerance uniformity, are difficult to control during wafer growth. The 808 nm diodes used to pump solid-state lasers typically have ±2 or 3 nm center wavelength tolerances: a tighter tolerance will increase the cost substantially. The supplier works from wafers with some nonuniformity. Entire units must be assembled and burned in before being screened to meet the wavelength specification. In this case, the supplier bases prices on costs relating to estimated yields. Remember, tight specifications translate to higher product costs. It is more cost-effective to provide for wavelength tuning by temperature than to require a specific wavelength.
Consider active alignment for tight positional tolerances— In general, passive alignment is desirable. But specifying positional tolerances of less than 100 µm results in two problems: alignment requires special fixturing, and the device usually requires a custom package (standard packages may have machining tolerances of up to ±125 µm). In many cases, active alignment can be more practical for systems with tight positional tolerances. Once again, tighter specifications mean higher product costs.
Don’t overspecify spatial output — Specifying the spatial output as power into an ∫ number of some full width half maximum is far less costly than specifying a shape. Don’t specify profiles or near-field patterns unless absolutely necessary. Many spatial characteristics are difficult for the manufacturer to control. This creates additional costs through the screening process and yield losses.
Specify only requirements that are important to the application — Unnecessary specifications always raise the final cost of the product because the manufacturer must test each quantitative specification for compliance.
Always verify operation using prototypes that cover the specification extremes — During the prototype test phase, it is not uncommon to discover that specifications need modifications. For example, the coupling efficiency through an optical system can often vary considerably from the theoretically calculated values. This type of production surprise is best discovered before total commitment.
Check a significant number of evaluation samples before committing to production — In one case, a customer bought one prototype device that worked well in his system. He then ordered 1000 diode lasers. After installing the first 100 in his system, he discovered that the tolerance for one parameter was set too wide, resulting in a 50 percent yield. A new specification required modifying the material. Besides paying for the new devices, the customer had to pay for those in his original order. Sufficient prototype testing could have prevented this problem.
Partner with your suppliers — Sharing proprietary information requires trust. Laser diode manufacturers are accustomed to handling proprietary information and, in many special laser applications, sharing of information is mutual. The supplier and the OEM must communicate freely to minimize costly errors.
Repeated iterations in developing custom devices are not uncommon. An OEM who understands the supplier’s position, communicates effectively with the supplier, and takes advantage of the supplier’s expertise will be able to make the custom development process as efficient and as cost-effective as possible.
Finding the Right Diode Supplier: The Importance of Flexibility
When contacting a supplier, be ready to explain what is special about your requirement. If the OEM product engineer isn’t sure what is needed, the supplier’s application engineer can help determine whether it is likely to involve customization and to what extent. The project may require wafer growth, spectral characteristics, packaging, and budgetary as well as environmental/thermal considerations.
Talking over a project is easier if each party understands the other’s thinking. The customer wants to get a job done. The diode manufacturer wants to figure out whether the customization is technically possible and whether the project is worth doing. A supplier may turn down projects that require a lot of work for little return or those that seem infeasible for other reasons.
The expertise of the supplier is important. What one may find infeasible or unpromising may seem worthwhile to another. Suppliers are aware of their own limitations and may turn down technically acceptable projects that don’t fit their expertise. Flexibility also plays a large role. Many companies are geared strictly for volume manufacturing operations and cannot easily accommodate custom projects.
Smaller suppliers often can provide a rapid response and may be willing to take on more specialized projects that larger manufacturers won’t handle. Flexibility and one-on-one service can be major advantages.
Volume is another factor in screening projects. Although there are large variations in product process, depending upon the volume, one can make some broad generalizations. If the project requires:
- One or two custom pieces for experiments, most manufacturers will turn down the job or charge a premium, making the project infeasible. The manufacturer will try to recuperate the costs of producing the custom product.
- Ten to 100 units in a one-time order, the manufacturer may be able to provide some customization at a more reasonable price. This volume, however, may still now allow the supplier to make a profit over the prototyping costs without making the final product price unreasonable.
- Ten to hundreds of prototype devices (with an expectation for full production later), custom wafer growth may be possible with costs amortized over prototype and production devices. Although custom package design is possible, standard packages are still more reasonable at this volume.
- Production quantities of hundreds to thousands of units over 12 to 18 months, custom wafer growth and custom packaging become economical.
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