Anna Filipchuk
D EVELOPMENT OF MEASUREMENT SETUP FOR CHARACTERIZATION OF HIGH - POWER INFRARED VCSEL ARRAYS
Bachelor of Science Thesis Faculty of Engineering and
Natural Sciences
Examiner: Associate Professor
Tapio Niemi
Examiner: D.Sc. Kostiantyn
Nechay
May 2021
ABSTRACT
Anna Filipchuk: “Development of measurement setup for characterization of high-power infrared Vertical-Cavity Surface-Emitting Laser arrays”
Bachelor of Science Thesis Tampere University
International Bachelor’s Degree in Science and Engineering (B.Sc. Tech.) May 2021
Laser technology market is rapidly developing as it provides innovative solutions to common problems in areas of ranging and surveying, communications, welding, and medicine. As the new types of lasers appear they require fast, simple, and versatile methods of characterization to conclude their competitive value.
Vertical-cavity surface-emitting lasers (VCSELs) are a particular type of semiconductor lasers, characterized by lasing happening perpendicular to the surface of the laser. This type of structural confinement leads to a remarkable set of characteristics they possess, which causes an increased interest in VCSEL development.
In this thesis the need for a measurement setup applicable for high-power IR VCSEL arrays characterization is addressed. The theoretical background behind laser operation is reviewed along with VCSELs specifications to identify key features of VCSELs to be considered in the measurement setup construction.
Prior to the experimental part of the research, the set of key parameters for laser array characterization is determined. They include: light-current-voltage curve (LIV-curve), beam profile, spectrum, number of lasing single emitters and temperature behavior characterization.
The fundamental methods for obtaining them are reviewed to define essential components of the future measurement setup.
Once all the components are identified the setup is constructed. The developed versatile setup provides the possibility of characterization of various VCSEL array parameters. The desirable combination of simplicity of operation along with the easiness of reconstruction between possible setup versions is achieved. The setup is further tested to validate accuracy and repeatability of the measurements performed. This is done by performing testing on commercial VCSEL arrays with beforehand known parameter values.
The further research could be dedicated to identifying the setup configuration, where all parameters of interest could be measured simultaneously without the need for setup reconfiguration. The optimal VCSEL array packaging which would provide the possibility for fast testing and ensure laser chip staying intact could also be investigated.
Keywords: semiconductor lasers, VCSEL array, laser characterization
The originality of this thesis has been checked using the Turnitin OriginalityCheck service.
PREFACE
This thesis was carried out at Modulight Inc. to be included as a part of a Bachelor’s Degree in Science and Engineering in Tampere University. The research was done in June-August 2020 and finalized in January-March 2021.
I would like to acknowledge my supervisors Prof. Tapio Niemi and Dr. Kostiantyn Nechay for their guidance and valuable advice. As well as express my gratitude to Modulight Inc.
for giving me an opportunity to discover the breath-taking world of laser technologies.
Finally, I would like to thank my family for their loyalty, support, and the ability to trust my decisions.
Tampere, 7 May 2021 Anna Filipchuk
C ONTENTS
1 INTRODUCTION ... 1
2 THEORETICAL BACKGROUND... 3
2.1 Basics of laser operation ... 3
2.2 Semiconductor lasers ... 5
2.2.1 VCSELs structure and design ... 5
2.2.2 VCSELs characteristics... 6
2.2.3 VCSEL arrays ... 8
3 LASER CHARACTERIZATION ... 10
4 SETUP DEVELOPMENT ... 12
4.1 Chip positioning ... 12
4.2 Number of lasing emitters measurements... 13
4.3 LIV and spectrum measurements ... 14
4.4 Beam profile measurements ... 15
5 RESULTS ANALYSIS ... 16
6 CONCLUSION ... 19
7 REFERENCES ... 20
L IST OF S YMBOLS AND ABBREVIATIONS
Abbreviations
AR augmented reality
ATR automatic target recognition COD catastrophic optical damage DBR distributed Bragg reflector
DC direct current
DFB distributed feedback
EEL edge-emitting laser
EM electromagnetic
HR high reflective
IR infrared
LD laser diode
LED light-emitting diode
LIV light-current-voltage
MBE molecular beam epitaxy
MOCVD metal organic chemical vapor deposition
ND neutral density
OSA optical spectrum analyzer
QW quantum well
VCSEL vertical-cavity surface-emitting laser
VR virtual reality
Symbols
𝜆 wavelength
𝐸 energy
ℎ Planck’s constant
𝑐 speed of light
𝐼 optical intensity
𝑃 optical power
𝜔 Gaussian beam radius
𝜃 half angle beam divergence
𝜔0 beam radius at beam waist
1 INTRODUCTION
It is impossible to overestimate the importance of laser technologies nowadays. They are present in almost all aspects of our lives and continue to broaden their application areas by being an innovative solution to a vast scope of problems. VCSEL is one example, where the laser type and its application areas exploration stimulate their mutual development.
VCSELs applications in communications, medicine, security, and sensing have secured them an essential place in our everyday life. Since the first VCSEL prototypes have been reported in 1965 by Ivars Melngailis [1] and after that the possibility of VCSELs fabrication proven experimentally in the works of Zh. I. Alferov (1975) and H. Soda (1979) [2], [3], over the past 40 years they have been gaining more and more commercial attention as an appealing alternative to common edge-emitting lasers (EELs). The reported increase in demand is partially due to the application fields of VCSELs, such as 3D sensing. Virtual reality/augmented reality (VR/AR) has developed especially rapidly over the past decade. Within the next few years the market is expected to triple its current size as seen from Figure 1.
Figure 1: Revenue forecast for VCSEL market 2018-2024 [4]
The increased interest in VCSELs is justified by the unique characteristics they possess.
The fundamental property – lasing perpendicular to the wafer surface and not parallel to it provides them with a number of features superior to EELs. Among those are low beam divergence, good temperature stability and possibility for 2D array production.
The fundamental differences between VCSELs and EELs would consequently require changes made to laser processing as well as testing and characterization. While EELs testing has become a common procedure the methods and techniques described for VCSEL characterization are quite limited.
This thesis seeks to address the development of a testing setup suitable for testing and characterization of high power VCSEL arrays lasing in infra-red (IR) region. The finalized setup should provide accurate and repeatable measurements for laser LIV-curve, optical spectrum and beam profile. The validity of the results is further justified by performing a series of measurements on commercial VCSEL arrays with the aim to conduct a comparative analysis of the array characteristics provided by the supplier to the ones obtained with the developed setup.
2 THEORETICAL BACKGROUND
To be able to better understand the techniques and methods of laser testing we should first look into laser structure and working principles. In this chapter the fundamentals of laser technologies are reviewed with a particular emphasis on VCSELs specifications.
2.1 Basics of laser operation
Laser is an acronym for “Light Amplification by Stimulated Emission of Radiation”. Unlike a common light bulb - a source of scattered, noncoherent and polychromatic light - laser light is monochromatic, coherent, collimated and has high intensity. All these properties are justified by the physical phenomena lying in the basis of its operational principles.
A simplified schematic of a laser cavity can be observed in Figure 2. Despite the type of laser we are considering, the three main components are: an active medium, resonator and pumping system.
Figure 2. Laser fundamental components
Active medium or gain medium is located inside the resonator and along with it defines the crucial characteristics of the laser output beam. To understand its working principle, we would have to consider the basic electrons recombination mechanisms.
In a two-level atomic system, the basic radiative electron recombination (Figure 3) include spontaneous recombination, characterized by electrons spontaneously dropping to a lower energy level and simultaneously emitting a photon. Despite the produced light being monochromatic, due to the inability to predict time or direction of the emitted photon this phenomenon cannot be used in laser diodes. It is however widely utilized in basic light-emitting diodes (LEDs) [5]. Another possibility is absorption or stimulated generation, which defines an opposite process, an electron absorbing the incident photon and raising to a higher energy level. And the last case is stimulated emission of radiation, upon which the laser operating principle is based. The incident photon
stimulates the emission of another photon, which when compensated for resonator losses, creates a positive gain mechanism. As it was demonstrated by Einstein - the produced photon’s frequency, direction, polarization and phase are identical to that of the incident electromagnetic (EM) wave [6]. When stimulated emission rate exceeds that of absorption – net optical amplification is achieved.
a) b) c)
Figure 3: Radiative electron recombination mechanisms [7]. (a) – spontaneous emission, (b) – stimulated generation, (c) – stimulated emission
The wavelength (𝜆) of the emitted radiation is defined with
𝐸 = ℎ𝑐
𝜆 , (1)
where 𝐸 is energy of the photon, ℎ is Planck’s constant, 𝑐 is speed of light.
As 𝐸 is defined by the difference in laser energy levels, which is a property of the atoms/molecules used, the laser wavelength depends on the material used as the gain medium. It can vary from 116 nm in hydrogen lasers to 10.6 m in carbon dioxide lasers [8]. Thus, laser emission starts in the IR part of the spectrum and goes up to ultraviolet (UV).
One of the ways of laser classification is based on the class of the gain material. The main groups are solid-state lasers (with semiconductor lasers being their subtype), gas lasers and fiber lasers.
The next element of the laser is a resonator. The resonator serves as a storage of coherent EM field and provides the possibility for its interaction with the gain medium. It consists of a reflector and partial reflector, positioned opposite and parallel to each other with the gain media being confined within them. Due to this structure, they are often described as Fabry-Perot resonators. The photons perform round trips between reflectors until the light leaves the laser through an output mirror. This way reduction of energy density within the resonator occurs [5].
The last component is the pumping system, which provides energy for electron absorption in the gain medium. Thus, net optical amplification is established. It should
be noted that pumping power must surpass the lasing threshold, which is the lowest excitation level after which spontaneous emission starts to be dominated by stimulated emission. Pumping photons have higher energy than the emitted ones. That is due to the fact that for achieving lasting condition where electrons are in the excited state, they often have to be excited to the pump level, which is a level higher than the desired excited state itself.
Based on the type of pumping system lasers can also be divided into various categories.
The pumping source can be: optical (lamp or another laser [9]), electrical current, chemical, nuclear reaction or gas discharge. Since the pumping system operates externally, it would be important to consider the type of pumping used when designing the characterization setup.
2.2 Semiconductor lasers
The semiconductor lasers can be named as the most ubiquitous group of lasers. The commonly used laser pointers, lasers utilized in CD players and optical fiber communications belong to this group. They can be classified as a part of a solid-state laser cluster due to the nature of the active media. In most cases semiconductor lasers are electrically pumped, however in certain cases optical pumping is applied instead.
VCSEL arrays used in this research are an example of electrically pumped semiconductor lasers. Their properties are reviewed in the following subsections.
2.2.1 VCSELs structure and design
Operating the theoretical background defined in the previous section we would now be able to characterize VCSELs. Laser diodes (LDs) are semiconductor lasers meaning the gain is provided by the flow of electrical current through p-n junctions. This flow causes the electron – holes recombination and consequent creation of photons. All LDs can be divided into two groups (Figure 4) based on the direction of the emission relative to the wafer surface. In the most common type, EELs, the emission takes place parallel to the wafer surface. Reflection takes place at the coated end facets of the semiconductor wafer which serve as mirrors. In VCSELs, however, emission takes place perpendicular to the wafer surface.
Figure 4: Types of lateral confinement in laser diodes [7]
Active region in VCSELs is thus clearly limited by the thickness of the semiconductor wafer, which is a thin slice of semiconductor [10] formed by highly pure and nearly defect free crystalline material. The active region is often realized as a quantum well structure (quantum well - 1D confinement, quantum wire – 2D, quantum dot – 3D). It is formed as a thin semiconductor medium embedded between two semiconductor layers of a material with higher band gap. For instance, InGaAs embedded in GaAs or GaAs embedded in AlGaAs. The thickness of a quantum well would normally lie in the range of 5-20 nm [11]. Such structures can be grown with epitaxy growth techniques, such as:
molecular beam epitaxy (MBE), characterized by sharp layers interfaces of high quality, or metal-organic chemical vapor deposition (MOCVD), which produces excellent quality of layers purity and crystallinity [12].
Another consequence of vertical confinement would be a rather short pathlength, which limits the round-trip laser gain. The resonator with low losses would thus be required.
Normally, distributed Bragg reflectors (DBR) are used as resonators in VCSELs. DBR is a mirror structure consisting of a sequence of layers of alternating two optical materials with high and low refractive indices. The design of the DBR is specific for each wavelength. Most frequently quarter-wave mirrors are used, meaning each optical layer thickness corresponds to ¼ of the laser emission wavelength [13]. The reflectivity of the mirror reaches up to 99%. The fact of DBRs being embedded into the wafer structure provides VCSELs with a set of superior traits in comparison to EELs.
2.2.2 VCSELs characteristics
The structure specifications discussed in the previous subsection lead to certain advantages of vertical emission structure. Firstly, monolithic wafer design i.e., the absence of the necessity for additional facet coating eliminates the need for waver cleavage before testing. This speeds up the processing, testing and overall development
process. The difficulty of handling chips with cleaved outer sides is also eliminated. The overall production costs are thus decreased significantly.
Another advantage lies in a superior laser emitting area. Its surface fully depends on the mask design and is not limited to the wafer thickness as in EELs. Combined with the generally lower power of a single VCSEL the optical intensity defined with
𝐼 = 𝑃 𝜋𝑤2
2
, (2)
where 𝐼 is optical intensity, 𝑃 is optical power, 𝑤 is Gaussian beam radius, is lower as well.
This eliminates the challenge of catastrophic optical damage (COD), which is a limitation factor for EELs caused by an overload in power density. The melting recrystallization of the semiconductor material at the laser facets is thus not a threat to VCSELs.
Lower optical intensities also enable operation at higher temperatures. Thus, the temperature sensitivity of the whole laser structure decreases. This eases the testing and operating condition requirements as well as increases the scope of application possibilities. Thin active area contributes to a narrow emission spectrum. This leads to the impossibility of several modes formation. Single mode operation is a valuable property required in various applications. As an example, they can be utilized for optical pumping in the cases where energy supplied by the laser should correspond very precisely to the energy gap of the pumped material.
The aperture design plays a key role in defining beam’s profile properties. Since the aperture is established completely by the mask design – it can be set to be circular in contrast to rectangular aperture in EELs (Figure 4). This ensures low beam divergence and high beam quality as seen from Figure 5. Moreover, the beam with such properties can be easier coupled to an optical fiber which increases its applicability.
Figure 5: VCSEL beam profile compared to LED and EEL [14]
Finally, one of the most appealing VCSELs properties contributing to their market popularity is the possibility to arrange them into 2D arrays. In this work we are going to assess the need for measurement setup for VCSEL arrays and thus we would take a look at their properties in more detail in the following subsection.
2.2.3 VCSEL arrays
Generally, despite VCSELs advantages, the power produced by a single emitter is quite limited and normally lies in the range of a couple of mW. The possibility of 2D arrays production, however, enables to increase the total output power to the order of 10 W [15].
Array formation techniques have already been applied to EELs (Figure 6), the power output of diode stacks composed of diode bars can reach the order of 100 W. However, their production is much more laborious and time consuming as it requires a sequence of consecutive processing steps.
a) b)
Figure 6: Schematic structure of (a) diode bar and (b) diode stack [16]
VCSELs 2D arrays production is much faster and more cost-effective. The produced power output is directly proportional to the number of single emitters in the array. In Figure 7 we can observe VCSEL arrays designed by II-VI Inc.
Figure 7: High Power VCSEL arrays (II-VI Inc.) [17]
While such qualities as high beam quality and low beam divergence apply to VCSEL single emitters, they are inversely proportional to the number of emitters in the array.
Meaning that by increasing the number of emitters with the aim of achieving higher power output we lose the beam quality. One of the possible solutions is to optimize the efficiency of the single emitters first and only then proceed to array formation. That way maximizing the power and simultaneously minimizing the number of single emitters used.
Another solution is to minimize the spacing between the single emitters since the area of the produced laser beam is higher than simply the sum of all single emitters areas due to the spacing being present. Spacing between single emitters defines pitch of the array.
While as small as possible spacing is desired due to higher beam quality, it might have a poor effect on the power output and efficiency conversion as the density of heat generation would be increased [18]. The optimal solution should thus be found to obtain the best possible efficiency with the lowest beam divergence simultaneously.
The efficiency of VCSEL arrays is comparable to that of EELs and reported to be about 50 %. The highest achieved efficiency for EELs is so far higher, cases of over 70 % have been reported [19], while the highest achieved value for VCSEL array is 63 % [20].
The possible beam profile divergence should be considered when performing laser testing. Beam collimation should be thus performed to avoid losses in power measurements. The absence of COD allows testing with high peak powers in pulsed operations.
3 LASER CHARACTERIZATION
To be able to carry out laser characterization to analyze laser properties and compare those against other devices on the laser market - testing has to be carried out. Testing and characterization are mutually connected since the testing setup construction is fully based on the properties we seek to analyze.
Main laser output parameters are normally displayed on the LIV-curve. The curve demonstrates the laser's power and output voltage as functions of the input current. Apart from power against current and power against voltage plots, it is a convenient way to conclude the laser’s threshold current and slope efficiency.
Another important point is the analysis of the laser spectrum conducted with an optical spectrum analyzer (OSA). The fundamental working principle of the device lies in dispersing the incoming light signal into components of different wavelengths and subsequent measurement of the intensity of each component (Figure 8). Different techniques can be used for wavelengths separation - one example would be the implementation of a monochromator, which utilizes the principle of diffraction grating.
Once the signal passes the monochromator it is received by the optical detector and is plotted on the display.
Figure 8: Grating spectrometer principle [21]
Generally, OSA response lies in the range of 800-1600 nm, which can be utilized for IR lasers studied without requirements for extra modifications.
With the help of laser beam profiler, it is possible to analyze such parameters as beam width, divergence, and astigmatism, characterized by elliptical light spots and hyperboloidal wavefronts [22]. It is also a way to estimate the beam quality, defined by beam quality factor 𝑀2. According to ISO 11146 standard [23], 𝑀2can be found with
𝑀2=𝜃𝜋𝜔0
𝜆 , (3)
where 𝜃 is half angle beam divergence, 𝜔0 is beam radius at beam waist, which is a direction along the propagation where beam radius obtains its minimum value.
Each beam profiler is suitable for a specific range of laser wavelength, power level and beam size. There exist various techniques and instruments for beam profiling, among which is a camera technique we are going to utilize in our research. The fundamental working principle is based on direct illumination of a camera sensor. As a result a 2D intensity plot (Figure 9) of the beam at measured location can be obtained.
a) b)
Figure 9: 2D intensity profiles of laser beams [24]
(a) Gaussian beam, (b) Multimode beam
To obtain a better understanding of laser behaviour in various circumstances - temperature characterization can be conducted. Thus, a laser chip is mounted on a thermal heat sink, the temperature of which can be varied. The temperature characterization is a useful addition to LIV and spectrum measurements. It provides us with more information on the spectral characteristics as well as an opportunity to observe nonlinearity of LIV behaviour signalizing of laser detuning.
Considering the type of laser chips to be tested an additional parameter would have to be taken into account. In the arrays it would be useful to know the number of working single emitters to be able to conclude on the estimated output power per emitter. This can be performed by positioning a camera to observe the lasing chip and afterwards analysing the obtained picture by the means of automatic target recognition (ATR).
4 SETUP DEVELOPMENT
In the current chapter we are going to focus on applying the theoretical background studied in the previous chapters to obtain a setup suitable for VCSEL array characterization. The desired setup should provide us with the possibility to measure all the characteristics discussed in the previous section. It should be taken into account that some of the measurements require beam focus on various equipment pieces. Thus, if such a contradicting situation occurs the setup should be easily rearranged to fit the required purpose.
4.1 Chip positioning
To be able to supply a steady current flow to the chip we should establish a proper closed circuit. From the structure of VCSELs we know that p contact is provided by p-side metallization of the top side of the chip and n contact by n-side metallization of the bottom. Several ways of chip positioning in the measurement setup have been reported [25], [26]. Once the chips are separated each array can be mounted and wired to a separate PCB, which itself is connected to the current driver via a basic connector [25].
However, if the wafer has not been yet cleaved or faster and more flexible testing is required the circuit can be established with probing needles [26]. One of the needles is connected to the top side, while the other is placed on the thermal heat sink as close to the testing piece as possible, as illustrated in Figure 10.
Figure 10: Schematics of VCSEL needle probe [26]
While this is a fast way to test chips still in the processing stage, it should be noted that probing needles might damage the chip surface. Thus, mass testing of chips in such a way after the development stage is concluded should be avoided.
4.2 Number of lasing emitters measurements
To analyze the number of emitters in the array, which contribute to the overall power output - we need to obtain the picture of the lasing array. Firstly, VCSEL emission direction suggests camera positioning perpendicular to the array surface. A diode is connected to the camera to provide the possibility to study the array surface when the array itself is not emitting light.
Secondly, once the VCSEL chip starts lasing - the intensity of the light hitting the camera tends to be too strong and no clear picture can be obtained. Neutral density (ND) filter is thus positioned between the beam and the camera providing optical attenuation in the chosen wavelength range. For VCSEL lasing in IR region, 650-1050 nm is an appropriate range. For obtaining higher attenuation levels, several filters might be combined.
The current is provided to the VCSEL array by the current driver. If the driver also has temperature control (TEC), it is connected to the heat sink, so that the temperature of the chip can be controlled throughout testing. The schematics of the setup can be seen in Figure 11. The camera is connected to the computer, where the image is observed and analyzed by the means of ATR.
Figure 11: Setup for measuring the number of working emitters
4.3 LIV and spectrum measurements
For obtaining accurate values for the LIV curve, divergence of the beam should be carefully considered since we have concluded that it is especially crucial for arrays due to the increase in divergence rate caused by their spatial arrangement. Thus, light from the LD is first collimated by passing through a plano-convex lens.
The aim of the setup construction is obtaining measurement versatility along with easiness of reconfiguration. Varying camera position from the setup setting described in the previous subsection would be too cumbersome, thus it would be optimal to insert a high-reflective mirror to change light direction. Then the light is collected by an integrating sphere, a device the interior of which is covered with a diffuse white reflective coating.
Light enters the sphere through a small hole and is afterwards distributed equally all over the inner surface of it [27]. Hence, though the spatial information about the beam is lost, the power is preserved. The photodetector inserted to the side of the integrating sphere and connected to the driver enables the measurements of the LIV curve. Simultaneously, spectrometer is connected to the integrating sphere. The setup shown in Figure 12 enables simultaneous measurements of the LIV curve and spectrum as well as temperature characterization of both.
Figure 12: LIV and spectrum measurement setup
To obtain the most precise measurements the beam position has to be calibrated. This can be done by either varying the LD or integrating sphere position. Since the LD position
is already calibrated with respect to the lens and the camera, adjusting the position of the integrating sphere is a preferable solution.
4.4 Beam profile measurements
For beam profile measurements, the integrating sphere would have to be changed to the camera beam profiler (Figure 13). The profiler is then connected to the computer and by utilizing the appropriate software the beam profile is mapped and analyzed. The adjustments of the temperature settings remain possible.
Figure 13: Beam profile measurement setup
5 RESULTS ANALYSIS
In the following chapter we highlight the measurement results we have managed to achieve by utilizing the developed setup. Further analysis addresses such properties as the simplicity of operation, easiness of reconstruction between different setup versions and obtained results validity and repeatability.
The final construction of the measurement setup can be observed in Figure 14. The lens- mirror system is attached via a magnet (Fig. 14 (a)), so that in case the setup needs to be changed to the version discussed in 4.2 it can be easily removed and changed to ND filters (Fig. 14 (b)).
a) b)
Figure 14: Setup for VCSEL array characterization
(a) Beam enters the integrating sphere, (b) Beam enters the camera
Once the mirror is removed and the beam has the possibility to enter the camera, we can estimate the number of working emitters. For that the current is gradually increased on the current driver until the threshold current is reached. Even before that we can observe spontaneous emission of light. The obtained pictures of the reference VCSEL
array can be observed in Figure 15 (a). Spontaneous emission starts at 18 mA, while lasing is detected at 200 mA. For capturing lasing ND filters system was utilized.
a) b)
Figure 15: (a) VCSEL array: spontaneous emission (left), lasing (right) (b) Lasing emitters mapped by the means of APR [28]
In the case when the VCSEL array is damaged and not all emitters are capable of lasing, APR methods are applied for detecting the number of working emitters. Such a case can be observed in Figure 15 (b).
For validating the accuracy of the measured LIV and spectra these parameters were measured for a reference VCSEL array from a commercial supplier. The data provided by the supplier was treated as reference and obtained results (Figure 16) were compared to it.
a) b)
Figure 16: (a) LIV curve and (b) emission spectrum for the reference VCSEL array
Before taking the measurements, calibrations of the integrating sphere, described in the previous chapter were performed. The highest measured power was 2.03 W, which precisely corresponds to 2 W reported by the supplier. At 20 C the spectrum peak was
detected at 942.94 nm, which lies in the range of 940 +/- 10 nm given in the laser chip datasheet.
The last modification of the developed setup implies the possibility of beam profile measurements. The obtained beam profile is presented in Figure 17. The observed beam pattern is single-mode and of close to Gaussian shape.
Figure 17: VCSEL array beam profile
The developed setup has been demonstrated to be straightforward in operation and modification. It also provides accurate results corresponding to the reference data. The results remain repeatable throughout the series of measurements, which enable further setup utilization for mass VCSEL arrays characterization.
6 CONCLUSION
An intense growth of VCSEL array market size is foreseen in the near future originating mostly from their superiority in the field of 3D sensing applications. This implies that the characterization methods and techniques of VCSEL array chips should be further developed.
In this thesis a comprehensive analysis of VCSEL arrays specifications is conducted and further applied to the characterization setup development. The versatility of the developed setup enables diverse properties of the VCSEL chips to be measured.
Characterization of the following properties is possible: LIV-curve, spectrum, temperature characterization, number of lasing emitters and beam profile.
The performance of each aspect of characterization was validated by testing reference VCSEL arrays with known characteristics. All measurements were repeated for a series of chips throughout which the consistency and repeatability of the results was demonstrated. The developed characterization setup provides the accuracy of measurements along with the simplicity of usage and possibility for versatile characterization of VCSEL lasing in IR region.
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