The market for laser diodes has recently experienced strong growth. The drivers of the enormous increase in demand are new applications and new technology concepts. Examples are face recognition in entertainment electronics, LiDAR (light detection and ranging) in automotive applications, and high-performance diode lasers that consist of several laser diodes and are increasingly replacing conventional laser systems in material processing.
Laser diodes can be subdivided technologically into EEL (edge-emitting laser) and VCSEL (vertical-cavity surface-emitting laser) (Figure 1). EELs emit horizontally to the chip surface. They cannot be optically characterized until the mirrored surfaces have been mounted on the lateral chip edges. By contrast, VCSELs — analogous to LEDs — are surface emitters and radiate perpendicular to the chip surface. The layer structure already contains the mirror required for the laser cavity. This results in a streamlining of the production process and permits optical characterization of the VCSEL already on the wafer. The production process of VCSELs compared to EELs is thus more efficient and more suitable for mass production.
The in-line quality control of laser diodes requires electrical as well as optical tests. Typically, a production test system inspects the light-current-voltage (LIV) curve and the optical spectrum. Here, “light” means more precisely the radiant flux of the laser diode in watts, often simply denoted as optical power. The LIV curve shows how the voltage over the VCSEL and the radiant flux varies with the driving current (Figure 2). This allows testing key performance parameters of the VCSEL, like the slope efficiency (η) and the threshold current (Ithr).
Test systems drive the VCSELs with current pulses in the microsecond to millisecond regime and monitor the light emission at the same time. The optical sensor is typically an integrating sphere (ISP) with a certain measurement system attached to it. Small ISPs with diameters of less than 100 mm are often preferred in production lines, because they increase the amount of light reaching the measurement system. This results in a higher throughput and is particularly important when testing weak light sources.
Radiant Flux Measurement
The minimum requirement for an inline optical measurement system for VCSELs is that it can determine the radiant flux. In the simplest case, this system can be a photodiode sensor attached to an integrating sphere. An absolute calibration of the photodiode with the ISP enables it to measure directly the optical power in watts and to determine the LIV curve. However, since the spectral sensitivity of photodiodes depends on the wavelength, production-related fluctuations of the VCSEL spectrum increase the error budget of the radiant flux measurement. This simple system cannot distinguish changes of the radiant flux from variations in the DUT spectrum. For standard and high quality products, radiant flux measurements alone are not sufficient. Besides the expected tight error budget, the spectrum and especially the peak wavelength also need to be controlled. Thus, spectral measurements are mandatory.
Scanning Spectral Measurement Systems
Established optical measurement methods for analyzing the spectrum of laser diodes are scanning measurement systems such as spectrum analyzers, Fourier transform infrared spectrometers (FTIR), and dispersive scanning spectrometers (Figure 3). Their mode of functioning is based on a scanning component, for example, a rotating diffraction grating for the scanning spectrometer or a movable mirror in the case of a FTIR. This design permits high spectral resolutions smaller than 0.1 nm accompanied by a flexible choice of spectral range to be measured. Such flexibility makes these measurement techniques highly effective for use in the laboratory, e.g. for product development and specification.
However, mass production of laser diodes, in particular VCSELs, calls for fast and at the same time accurate and comparable in-process inspection during which each individual laser diode is checked. In this kind of production environment, scanning measurement methods have certain limitations: production related vibrations may impair the positioning accuracy of moving components and, thus, the stability and repeatability of measurement results. The duration of measurement is at least the length of the scanning process, thus severely limiting throughput volume. Spectral measurement requires constant optical performance of the laser diode during scanning to avoid distortion of the spectrum. Scanning systems are therefore unsuitable for the measurement of laser diodes with a pulsed operating mode.
Array Spectral Measurement System
In comparison, array spectroradiometers (Figure 4) - long-established as a measurement solution in LED production - are ideal for use in the production of laser diodes. The fixed configuration of the optical components ensures a high degree of mechanical stability and high repeat accuracy of optical measurements. The spectrum is measured in an instant using an array detector. This enables integration times of just a few milliseconds for CCD detectors and microseconds for CMOS detectors, and an extremely high throughput rate.
Spectral measurements of laser diodes with very short pulses are therefore possible, which is also a frequent use case for array spectroradiometers in the laboratory. In addition, an absolutely calibrated spectroradiometer delivers the performance as an immediate reading, so that no additional photodiode sensor is required for power measurement. Spectral resolutions in the range of 0.1 to 0.2 nm can be achieved with a suitable choice of optical components (Figure 5).
Many of the novel applications for VCSELs, especially in 3D sensing, require much more optical output power than a single VCSEL emitter can generate. In this case, VCSEL arrays are a good choice. These devices can consist of several hundred single emitters on the size of only a few square millimeters and can generate several watts of optical power with a bandwidth of about one nanometer.
Two spatial regimes of the VCSEL emission pattern can be distinguished: In the so-called far field, the emission patterns of the individual emitters overlap and cannot be separated anymore. In contrast, the near field is the space in which the single apertures on the device are resolved and can be studied.
VCSEL arrays have additional needs for testing in production lines. Besides the optical spectrum and the light-current-voltage (LIV) curve, the shape and the numerical aperture of the far field radiation pattern, and the number and location of defect emitters on the chip are also important to measure (Figure 6). This requires additional production-grade spatial measurement equipment that offers high-stability and throughput at the same time. Commonly, the angle-dependent spatial radiation pattern of a light source is characterized with a goniophotometer. However, this very accurate method takes too long for sample inspection in production. Faster alternatives are either conoscopes or camera-based imaging systems, which observe the image of the light source on a screen. Independent of the exact system, its calibration is highly important for accurate optical measurements. All systems containing a camera require at least a flat-field calibration that takes care of artificial brightness variations of the image due to the measurement system. Further, an absolute calibration of the near-field measurement system is necessary if the radiant flux per emitter on the array is a target value.
Nanosecond Pulses Testing
VCSEL arrays for time-of-flight applications typically require high optical peak powers in the range of several watts and driving with nanosecond pulses to increase the spatial resolution and the maximum detection range within the eye safety limits. In this case, in-line quality control is especially demanding because the VCSELs need to be tested with nanosecond-pulses to verify proper fabrication, bonding and packaging, since parasitic capacitances and inductances can deteriorate short-pulse operation. For this reason, an additional fast photodiode sensor capturing the nanosecond pulse train completes the optical test system. A challenge is the electrical driver for in-line nanosecond VCSEL inspection, because it must supply very high peak currents (many ampere) in just a few nanoseconds to the device-under-test.
A traceable, absolute radiant flux calibration is necessary to specify a reliable error budget. This is vital because the working range of devices based on the time-of-flight principle scales with the emitted peak power of the laser diode. However, its maximum value is restricted by the limits for eye-safe operation. Therefore, a traceable, absolute calibration of the radiant flux measurement guarantees VCSELs that fulfill all laser safety requirements, but also maximize the working range of the system.
Array spectroradiometers satisfy the exacting requirements for spectral resolution, throughput volume, and reliability in a production environment. They are thus ideally suited to optical in-process inspection of laser diodes. In addition, array spectroradiometers can be used for the measurement of pulsed operation laser diodes and are an effective measurement method for use in the laboratory, too. The rise of VCSEL arrays imposes new challenges on production testing. Especially, spatial measurements in the near and far field of a VCSEL array are required. Camera-based imaging systems and conoscopes are suitable for production testing of spatial VCSEL characteristics. In-line testing of VCSELs with nanosecond pulses will become more and more relevant with their increasing spread in time-of-flight applications.
This article was written by Dr. Martin Finger, Application Engineer, Instrument Systems GmbH (Munich, Germany). For more information, contact Dr. Finger at