Princeton Optronics is the technology leader in high power VCSEL diode laser technology and in the area of diode laser pumped low noise laser technology. The technology section write-up is organized as follows:
VCSEL devices and modules for pumping
High speed VCSEL devices
Frequency doubled and quadrupled diode lasers for Blue, green and UV output for illumination, entertainment and projection systems
VCSEL pumped solid state lasers for military and commercial applications
- Application of VCSELs
- Summary of key properties of Princeton Optronics VCSELs
- Introduction to VCSEL didode laser technology
- High Power CW and QCW VCSEL diode laser arrays
- High temperature operation of the VCSEL diode lasers
- VCSEL diode laser reliability
- Single mode VCSEL diode lasers for atomic clock, high speed and sensing applications
- Red VCSELs for medical and other applications
- Resonant cavity LEDs for OCT and low specle applications
- VCSEL diode laser illuminator modules and chips for various applications
I. Applications of Princeton Optronics VCSELs:
Princeton Optronics high power VCSELs are used for the applications which are:
- Illumination for silicon camera for high speed, low light level and other applications such as 3D and gesture recognition (for products in this area click on Illumination products).
- VCSEL pumps for solid state lasers (for products in this area click on Pump products).
- Single mode VCSELs for atomic clock, high speed and sensor applications (for products in this area click on Single mode VCSEL products).
- Red VCSELs for medical and illumination applications (for products in this area click on Red VCSEL products).
- Resonant cavity LEDs for short coherence length applications such as OCT and very low speckle illumination. (for products in this area click on Resonant Cavity LED or RC-LED products).
- Frequency doubled lasers in blue and green wavelengths (for products in this area click on Frequency doubled products).
- VCSEL pumped solid state lasers in blue, green and UV wavelengths (for products in this area click on Diode pumped solid state laser products).
II. Summary of Key Properties of Princeton Optronics VCSELs:
The following table summarizes the key properties and products of Princeton Optronics VCSELs:
||Princeton Optronics VCSEL Status Today
||Comments on VCSELs |
|Power and Power density of arrays
||1kW/ cm2 CW; 4.2kW/ cm2 QCW as they are be close packed
||CW power from a single chip is 200W/cm2 (max). More commonly it is about 100W from a 5x5mm chip mounted on a submount. For QCW, max power from a single chip is >800W from a 5x5mm chip |
||Max is ~ 62% for CW. Commercial shipments are >50%
||High temp efficiency -55% at 50 deg C and 50% at 60 deg C and 45% at 65 deg operation demonstrated. |
||Circular beam and the beam divergence is ~19 deg (1/e2). This is true for single devices and arrays.
||Beam divergence is lower for certain types of devices for example, extended cavity single mode devices where the divergence is ~4 deg. The divergence can be tailored. |
|Beam profile and fiber coupling
||The beam is circular and the beam profile is flattop in nature.
||The circular beam lends into good fiber coupling. We typically get >:90% efficiency for coupling into fiber. |
|Wavelengths offered for single devices and arrays
||473nm, 530nm, 650nm (red), 780nm, 795nm, 808nm, 830nm, 850nm, 976nm and 1064nm
||473 and 530nm are frequency doubled VCSELs, Custom wavelengths are offered. |
||95 deg C extended operation demonstrated PRO, >4 million device hrs at 80 deg C operation recorded
||Efficiency at high temp as well as overall efficiency is improving with time. Please check this website for latest numbers. |
|Wavelength spread and temp dependence
||0.06nm/deg temp dependence, 0.8nm wavelength spread for a 100W or 200W array.
||Can have lax temp control- less cooling need. |
||In certain configurations, the VCSELs can be used as a class A or class 1 laser device.
||Eye safety can be improved for VCSEL arrays because it is an extended source. |
||VCSEL arrays have very low speckle, almost same as LEDs. Speckle is further improved by certain modifications of the array.
||Speckle is very important for a number of applications related to imaging. We have a number of tools at our disposal to improve the speckle for very low noise speckle laser applications. |
||We offer the VCSELs in chip on submount, TO, and multi-chip module formats.
||For special applications we offer fiber coupled formats as well. Consult us for any special packaging. |
|Single Device Reliability (FIT-failure in billion device hours.
||FIT~10-3 without ESD- by many companies- >10E5 times better than Edge Emitters.
||We have array reliability data- only company with such comprehensive data for arrays. |
III. Introduction to Vertical-Cavity Surface-Emitting Diode Laser (VCSEL) Technology
Vertical-Cavity Surface-Emitting Diode Lasers (VCSELs) are a relatively recent type of semiconductor lasers. VCSELs were first invented in the mid-1980's. Very soon, VCSELs gained a reputation as a superior technology for short reach applications such as fibre channel, Ethernet and intra-systems links. Then, within the first two years of commercial availability (1996), VCSELs became the technology of choice for short range datacom and local area networks, effectively displacing edge-emitter lasers. This success was mainly due to the VCSEL's lower manufacturing costs and higher reliability compared to edge-emitters.
Princeton Optronics has developed the key technologies resulting in the world's highest power single VCSEL diode laser devices and 2-D arrays. We have successfully demonstrated single devices with >5W CW output power and large 2D arrays with >230W CW output power. We have made single mode devices of 1W output power and single mode arrays with power of >100W which are coupled to 100u, 0.22NA fiber. The highest wall plug efficiency of these devices and arrays is 56%. We have made arrays which deliver 1kW/cm2 in CW operation and 4.2kW/cm2 in QCW operation. Princeton Optronics was a participant in the DARPA-SHEDS program, whose main objective was to improve laser diode power conversion efficiency.
The VCSEL Diode Laser structure
Semiconductor diode lasers consist of layers of semiconductor material grown on top of each other on a substrate (the "epi"). For VCSELs and edge-emitters, this growth is typically done in a molecular-beam-epitaxy (MBE) or metal-organic-chemical-vapor-deposition (MOCVD) growth reactor. The grown wafer is then processed accordingly to produce individual devices. Figure 1 summarizes the differences between VCSEL and edge-emitter processing.
Figure 1. Comparison of the growth/processing flow of VCSEL and edge-emitter semiconductor lasers. VCSEL processing is very similar to processing of LEDs.
In a VCSEL, the active layer is sandwiched between two highly reflective mirrors (dubbed distributed Bragg reflectors, or DBRs) made up of several quarter-wavelength-thick layers of semiconductors of alternating high and low refractive index. The reflectivity of these mirrors is typically in the range 99.5~99.9%. As a result, the light oscillates perpendicular to the layers and escapes through the top (or bottom) of the device. Current and/or optical confinement is typically achieved through either selective-oxidation of an Aluminum-rich layer, ion-implantation, or even both for certain applications. The VCSELs can be designed for "top-emission" (at the epi/air interface) or "bottom-emission" (through the transparent substrate) in cases where "junction-down" soldering is required for more efficient heat-sinking for example. Figure 2 illustrates different common types of VCSEL structures.
In contrast, edge-emitters are made up of cleaved bars diced from the wafers. Because of the high index of refraction contrast between air and the semiconductor material, the two cleaved facets act as mirrors. Hence, in the case of an edge-emitter, the light oscillates parallel to the layers and escapes side-ways. This simple structural difference between the VCSEL and the edge-emitter has important implications.
Figure 2. Three common types of VCSEL diode laser structures: (a) a top-emitting structure with proton implantation to confine the current, (b) a selectively-oxidized top-emitting structure to confine the optical modes and/or the current, and (c) a mounted bottom-emitting selectively-oxidized structure.
Since VCSELs are grown, processed and tested while still in the wafer form, there is significant economy of scale resulting from the ability to conduct parallel device processing, whereby equipment utilization and yields are maximized and set up times and labor content are minimized. In the case of a VCSEL (see Figure 1), the mirrors and active region are sequentially stacked along the Y axis during epitaxial growth. The VCSEL wafer then goes through etching and metalization steps to form the electrical contacts. At this point the wafer goes to test where individual laser devices are characterized on a pass-fail basis. Finally, the wafer is diced and the lasers are binned for either higher-level assembly (typically >95%) or scrap (typically <5%). The following Figure 3 shows a single high-power VCSEL diode laser device (>2W output power) packaged on a high-thermal conductivity submount. Fig 3(a) shows the L-I characteristics of a 5W VCSEL diode laser device.
Figure 3. Packaged high-power VCSEL device (>2W). The submount is 2mm x 2mm.
Fig 3(a). L-I characteristics of a 5W VCSEL diode laser device. The device aperture is 300u.
In a simple Fabry-Pérot edge-emitter the growth process also occurs along the Y axis, but only to create the active region as mirror coatings are later applied along the Z axis. After epitaxial growth, the wafer goes through the metallization step and is subsequently cleaved along the X axis, forming a series of wafer strips. The wafer strips are then stacked and mounted into a coating fixture. The Z axis edges of the wafer strips are then coated to form the device mirrors. This coating is a critical processing step for edge-emitters, as any coating imperfection will result in early and catastrophic failure of the devices due to catastrophic-optical-damage (COD). After this coating step, the wafer strips are diced to form discrete laser chips, which are then mounted onto carriers. Finally, the laser devices go into test.
In addition to these advantages, VCSEL also demonstrate excellent dynamic performances such as low threshold currents (a few micro-amps), low noise operation and high-speed digital modulation (10 Gb/s). Furthermore, although VCSELs have been confined to low-power applications - a few milli-Watts at most - they have the inherent potential of producing very high powers by processing large 2-D arrays. In contrast, edge-emitters cannot be processed in 2-D arrays.
VCSEL Diode Laser Advantages
The many advantages offered by the VCSEL diode laser technology can be summarized in the following points:
1. Wavelength stability: The lasing wavelength in a VCSEL diode laser is very stable, since it is fixed by the short (1~1.5-wavelength thick) Fabry-Perot cavity. Also, because of the short cavity, the longitudinal modes are far apart (~40nm) so that mode hopping between different longitudinal modes is not likely.
2. Wavelength uniformity & spectral width: Growth technology has improved such that VCSEL 3 or 4” wafers are produced with less then a 2nm standard deviation for the cavity wavelength. This allows for the fabrication of VCSEL diode laser 2-D arrays with little wavelength variation between the elements of the array (<1nm full-width half-maximum spectral width). By contrast, edge-emitter bar-stacks suffer from significant wavelength variations from bar to bar since there is no intrinsic mechanism to stabilize the wavelength, resulting in a wide spectral width (3~5nm FWHM).
3. Temperature sensitivity of wavelength: The emission wavelength in VCSEL diode lasers is ~5 times less sensitive to temperature variations than in edge-emitters. The reason is that in VCSELs, the lasing wavelength is defined by the optical thickness of the single-longitudinal-mode-cavity and that the temperature dependence of this optical thickness is minimal (the refractive index and physical thickness of the cavity have a weak dependence on temperature). On the other hand, the lasing wavelength in edge-emitters is defined by the peak-gain wavelength, which has a much stronger dependence on temperature. As a consequence, the spectral line-width for high-power arrays (where heating and temperature gradients can be significant) is much narrower in VCSEL arrays than in edge-emitter-arrays (bar-stacks). Also, over a 20oC change in temperature, the emission wavelength in a VCSEL will vary by less than 1.4nm (compared to ~7nm for edge-emitters).
4. High Temperature Operation (Chillerless operation for pumps): VCSEL diode laser devices can be operated without refrigeration- because they can be operated at temperatures to 80 deg C, The cooling system becomes very small, rugged and portable with this approach. We are getting good efficiency at higher temperatures as well. For example, we are getting array efficiency of >55% at a temp of 50 deg C and an efficiency of >45% from a high power arrays at a temperature of 65 deg C. Princeton Optronics VCSELs are being used for pumping for at 80 deg C by some customers.
5. Higher power per unit area: Edge emitters deliver a maximum of about 500W/cm2 because of gap between bar to bar which has to be maintained for coolant flow, while VCSEL diode lasers are delivering ~1200W/ cm2 now and can deliver 2-4kW/ cm2 in near future. We obtain a power output of >900W from a single 5x5mm VCSEL at some wavelengths.
6. Beam Quality: VCSELs emit a circular beam. Through proper cavity design VCSEL diode lasers can also emit in a single transverse mode (circular Gaussian). This simple beam structure greatly reduces the complexity and cost of coupling/beam-shaping optics (compared to edge-emitters) and increases the coupling efficiency to the fiber or pumped medium. This has been a key selling point for the VCSEL diode laser technology in low-power markets.
7. Reliability: Because VCSELs are not subject to catastrophic optical damage (COD), their reliability is much higher than for edge-emitters. Typical FIT values (failures in one billion device-hours) for VCSEL diode lasers are <10.
8. Manufacturabilty and yield: Manufacturability of VCSEL diode lasers has been a key selling point for this technology. Because of complex manufacturing processes and reliability issue related to COD (catastrophic optical damage), edge-emitters have a low yield. The VCSELs can be tested on wafer reducing their testing cost. In fact, because of its planar attributes, VCSEL diode laser manufacturing is identical to standard IC Silicon processing. For some applications, VCSEL prices are becoming comparable to LED pricing. Princeton Optronics has a very high volume manufacturing operation.
9. Scalability: Mounting of large high-power VCSEL 2-D arrays in a “junction-down” configuration is straightforward (similar to micro-processor packaging), making the heat-removal process very efficient, as the heat has to traverse only a few microns of AlGaAs material. Record thermal impedances of <0.16K/W have been demonstrated for 5mm x 5mm 2-D VCSEL arrays. The VCSEL arrays can be used with surface mount packaging much like silicon devices as well as LED arrays.
10. Packaging and heat-sinking: Mounting of large high-power VCSEL 2-D arrays in a “junction-down” configuration is straightforward (similar to micro-processor packaging), making the heat-removal process very efficient, as the heat has to traverse only a few microns of AlGaAs material. Record thermal impedances of <0.16K/W have been demonstrated for 5mm x 5mm 2-D VCSEL arrays. The VCSEL arrays can be used with surface mount packaging much like silicon devices as well as LED arrays.
11. Cost: With the simple processing and heat-sinking technology it becomes much easier to package 2-D VCSEL arrays than an equivalent edge-emitter bar-stack. The established existing silicon industry heat-sinking technology can be used for heat removal for very high power arrays. This significantly reduces the cost of the high-power modules. The cost of VCSEL devices and arrays are steadily coming down for high volume applications.
Excellent Wavelength Stability and Low Temperature Dependence
Since the VCSEL resonant cavity is defined by a wavelength-thick cavity sandwiched between two distributed Bragg reflectors (DBRs), devices emit in a single longitudinal mode and the emission wavelength is inherently stable (<0.07nm/K), without the need for additional wavelength stabilization schemes or external optics, as is the case for edge-emitters. Furthermore, thanks to advances in growth and packaging technologies, the emission wavelength is very uniform across a 5mm x 5mm VCSEL array, resulting in spectral widths of 0.7~0.8nm (full-width at half-maximum). This wavelength stability and narrow spectral width can be very significant advantages in pumping applications for example where the medium has a narrow absorption band.
Figure 4. Emission spectrum of a 5mm x 5mm VCSEL array at 100W output power
Circular output beam
Unlike edge-emitters, and like LEDs, VCSELs emit in a circularly symmetric beam. The VCSEL divergence is about 20 degrees (1/e2) for single devices and arrays. The divergence can be increased to any level by use of external optical elements. This has been a tremendous advantage for low-power VCSELs in the telecom, datacom and sensing markets because of their ability to directly couple to fibers (“butt-coupling”) with high coupling efficiency. Princeton Optronics’ high-power VCSEL arrays emit in a quasi-top-hat beam profile, making these devices ideal for direct pumping (“butt-pumping”) of solid-state lasers.
Figure 5. Far-field beam profile of a 5mm x 5mm VCSEL array at 100W output power
IIn VCSELs, the as-grown output coupler reflectivity is very high (typically >99.5%) compared to edge-emitters (typically <5%). This makes VCSELs extremely insensitive to optical feedback effects, thus eliminating the need for expensive isolators or filters in some applications.
Low thermal impedance & ease of packaging
Princeton Optronics has developed advanced packaging technologies, which enables efficient and reliable die-attach of large 2-D VCSEL arrays on high-thermal-conductivity submounts. The resulting submodule layout allows for straightforward packaging on a heat-exchanger.
For high-power devices packaged on micro-coolers, Princeton Optronics has demonstrated modules with thermal impedances as low as 0.15K/W (between the chiller and the chip active layer). Princeton Optronics can provide its customers with several heat-exchanger and heat-sinking application notes.
IV. High Power CW and QCW VCSEL Diode Laser Arrays:
Princeton Optronics designs and manufactures advanced high-power CW and QCW diode lasers for the industrial, medical, and defense markets. Princeton Optronics' innovative approach is based on the Vertical-Cavity Surface-Emitting Laser technology (VCSEL for short), enabling us to manufacture and deliver laser diodes with exceptionally high reliability, and superior spectral and beam properties.
Arrays with Hundreds of Watts (>1kW/cm2 Power Density) Output Power:
Vertical-Cavity Surface-Emitting Lasers were initially introduced in the mid-90’s as a low-cost alternative to edge-emitters, for use as a low-power source (sub-mW to a few mW) in datacoms and telecoms. Within two years of their introduction, VCSELs overwhelmed and replaced the edge-emitter technology in these markets due to their better beam quality, reduced manufacturing costs and much higher reliability. Now, a new class of VCSELs has been developed for high power applications. Princeton Optronics is the first company to introduce such high power VCSEL products to the market.
Unlike edge-emitters, the light emits perpendicular to wafer surface for VCSELs. It is therefore straightforward to process 2-D arrays of small VCSEL devices driven in parallel to obtain higher output powers. The advantage of 2D arrays is that it has simple silicon IC chip-like configuration and many of the silicon IC packaging and cooling technology can be applied to VCSEL arrays.
Princeton Optronics has taken the VCSEL technology to very high power levels by developing very large (5mm x 5mm) 2-D VCSEL arrays packaged on high-thermal-conductivity submounts. These arrays are composed of thousands of low-power single devices driven in parallel. Using this approach, record CW output powers in excess of 230W from a 0.22cm2 emission area (>1kW/cm2) have been demonstrated, without sacrificing wall-plug efficiency.
Figure 6. Picture of high-power 5mm x 5mm 2-D VCSEL array mounted on a micro-cooler and measure CW output power and voltage at a constant heat-sink temperature. Roll-over power is >230W.
Very high-power density QCW operation
In addition to CW VCSEL diode laser arrays, Princeton Optronics has developed very high power density VCSEL arrays for quasi-CW (QCW) operation. QCW powers in excess of 925W have been demonstrated from very small arrays (5x5mm chip size), resulting in record power densities >4.2kW/cm2. These small arrays can easily be connected in series to form larger arrays with high output powers. These arrays are ideal for applications requiring very compact high-power laser sources.
Figure 7. Power vs. current for a small VCSEL 2D array under different QCW regimes. These arrays exhibit power densities >4.2kW/cm2.
V. High Temperature Operation of the VCSEL Diode Lasers
Because VCSELs can operate reliably at temperatures up to 80 oC, they do not necessarily require refrigeration. Additionally, since the wavelength change with temperature is small, the cooling system design can be considerably simplified. The cooling system thus becomes very small, rugged and portable with this approach. We have been operating the VCSELs and VCSEL arrays with water pump and a radiator cooling like that of a car engine. Fig 8 shows such a set up in which a radiator and water pump is used to cool a 120W array of VCSELs. The result of the cooling arrangement is compared with a chiller cooling and shown in Fig 9.
Figure 8. Shows the set up without chiller using a radiator and a water pump in an arrangement like in a car engine.
Figure 9A. Shows the performance of a 120W VCSEL diode laser array with fan-radiator cooling with water temperature at 45 deg C vs cooling with a chiller with 16 deg C water temperature. The green curves show the efficiency (CE) in the two cases which is almost similar. The red curves show the power output from the array in the two cases. The power output decreases somewhat at higher power, but at power levels below 80W, there is very little change.
Figure 9B. Shows the L-I curves for a diode laser at 976nm at 20 and 50 deg C. The top curve (blue)is at 20 deg C and the bottom one (red) is at 50 deg C. The efficiency at 20 deg C is 60% and at 50 deg C it is 55%.
VI. VCSEL Diode Laser Reliability
In terms of reliability, VCSELs have an inherent advantage over edge-emitters because they are not subject to catastrophic optical damage (COD). Indeed, the problem of sensitivity to surface conditions for edge-emitters is not present in VCSELs because the gain region is embedded in the epi-structure and does not interact with the emission surface.
Over the years, several reliability studies for VCSELs have yielded FIT rates (number of failures in one billion device-hours) on the order of 1 or 2, whereas FIT rates for the highest telecom-grade edge-emitters is on the order of 500. The failure rate for industry-grade high-power edge-emitter bars or stacks is even worse (>1,000).
Princeton Optronics has accumulated millions of device-hours on VCSELs operating above 100C (Fig. 10).This reliability advantage will be very significant for laser systems, where the end-of-life and field failures are overwhelmingly dominated by pump failure. Moreover, VCSEL arrays can be operated at higher temperatures, resulting in lower power consumption of the overall laser system. Princeton Optronics has demonstrated reliable operation up to 80oC. For more detailed information on reliability, please contact Princeton Optronics sales.
Figure 10. Single VCSEL accelerated aging cell showing 29,600 hours (almost three and a half years) of continuous operation at high temperature (~100oC) for several devices.
VII. Single Mode VCSEL Diode Laser Devices:
We make single mode devices with power level of 5mW for small aperture devices (4u) as well as higher power devices of power output of 150mW from a 100u aperture devices in TO can packages and upto 1W of a single device in a larger package. Some of the the devices have narrow linewidth of tens of kHz and SMSR of >30dB. The devices have high efficiency of >40%. Fig 11 shows the L-I characteristics as well as SMSR of a single mode low power device.
The single mode devices are finding volume applications in sensing and atomic clock for navigation. Atomic clock devices are fabricated at wavelength of 780.24 and 795nm. Princeton Optronics makes self lasing devices which have a power level of 3-4mW and extended cavity devices of ~15mW power with linewidth in the range of ~1MHz.
Fig 11. (top left) L-I characteristic of a 5u aperture VCSEL device. (top right). Is the SMSR of the device. (bottom). High temperature performance of a single mode device at temperatures upto 60 deg C. The power conversion efficiency (PCE) is 30% at 60 deg C and the power is >4mW at that temperature. At 95 deg C the power goes down to 3mW but the efficiency is still >25%.
12(a). L-I curve for a 780nm single mode atomic clock laser. The linewidth is <1MHz, the power is >15mW. The laser is available in a TO can.
Fig 12(b). L-I curve for a 795nm single mode atomic clock laser. The power is >3mW. The laser is available in a TO can. The inset shows the spectrum of the laser output.
VIII. Red VCSELs for Medical, Illumination and Printing Applications:
Currently, we are making red VCSELs at 650nm and at other wavelengths for medical and illumination applications. Fig xx shows the picture of a 650nm 10W VCSEL array on submount. The beam is incident on a conducting screen on top. We have products of 2W and 10W at 650nm in the red. We can make custom addressable arrays and have made custom and addressable arrays for special applications.
Fig 13. The picture of a 10W VCSEL array at 650nm mounted on a submount. The beam is incident on the inclined screen
IX. Resonant Cavity LEDs for Short Coherence Length applications such as OCT(Optical Coherence Tomography) and Some Very Low Speckle Applications:
VCSEL structure is very similar to the LEDs. Resonant cavity LEDs are made with one mirror of VCSEL with high reflectivity, but the other mirror being low reflectivity. The beam is directed so that it can be coupled with fibers, but it does not laze. For that reason a wavelength spread of 5nm or more can be obtained depending on output mirror reflectivity and the coherence length can be very short, similar to the LEDs. Such devices are needed for many applications such as OCT (Optical Coherence Tomography) and other applications. We have a product at 976nm which is selling in reasonable volume.
X. VCSEL Diode Laser Illumination modules for high resolution 3D imaging, gesture recognition and high speed imaging:
Princeton Optronics has developed VCSEL arrays of 3, 6,15 and 50W power output chips which can be used for illuminator applications. These chips will be replacement of the LED chips which are used for illuminators with silicon CCD or CMOS cameras. The illumination wavelengths are at 808, 976 and 1064nms. The VCSEL chips have a divergence of about 16 deg full angle compared to a very wide divergence of the LED chips. VCSELs have much higher efficiency (50%) vs an efficiency of ~10% for the LED devices.
Output profile from the Illuminator Chips:
The output profile of the illuminator chips can be seen in the next figure (Fig 14). The near field image can be projected by a projection system. The far field generally overlaps into a circular beam.
Fig 14. The output profile from a VCSEL array for illumination. The VCSEL chip can be seen on left, the near field image in the middle and a far field circular beam on the right.
Speckle free Illumination:
Illumination from a VCSEL diode laser array is very low speckle. Measurement have shown that the illuminated area has speckle of <1%. Therefore VCSEL illumination is very desirable for many applications where uniformity of illumination is very important.
Fig 15. Example of speckle from edge emitter (left) vs VCSEL array (right). Speckle is very important for imaging as well as for applications where low noise from the image is required.
Fast Rise and Fall Times from Arrays for Illumination:
In case of illuminators for some applications, fast rise and fall times are important, such as for Time of Flight (TOF) measurement. Such rise and fall times for a VCSEL array can be seen in Fig 16. The rise and fall times for VCSEL arrays are generally low because of low RC constant for the arrays.
Fig 16 . The rise and fall time of a VCSEL array. They are typically below <300ps.
Eye Safety from VCSEL Illumination:
Eye safety is a major concern for commercial applications. Because of the extended illumination area involving a very large number of individual devices the human cornea does not focus the light into a single spot and therefore the Maximum Permission Exposure (MPE) for VCSEL arrays are considerably higher than edge emitting lasers. Fig 17 gives a schematic of eye exposure scenario where human cornea focuses the illuminating radiation from VCSELs. Using certain optical elements one can make the VCSELs suitable for class A or class 1 operation.
Fig 17. A schematic of how human eye focuses the laser illumination from a VCSEL into the retina. Because of the extended nature of the emission area, the MPE (Maximum Permissible Exposure) for a VCSEL array is much higher than comparable edge emitting or other lasers. For more details on MPE contact Princeton Optronics.
Illuminator Chips Usable for Many Applications:
Princeton Optronics sells illuminator chips to the companies who make illuminator products for many industrial and commercial applications. The chips have power levels from <1W to several hundred watts/chip. Please contact us for your needs.
Fig 18. Illumination for 3D imaging and gesture recognition. There are many technologies for gesture recognition such as Time of Flight (TOF- on top) in which multiple pulses of light are emitted from the source and the time delay between the emitting pulses and its reflected return. The other laser technique is called structured lighting (left) in which a single laser beam is split into multiple beams which will be incident on the target and depending on the size and location of them, the depth can be ascertained. Both of these approaches need high quality lasers and VCSEL devices are good illuminators for both approaches.
Commercial VCSEL Diode Laser Illuminator Chips and Modules:
Illuminator Chips for Different Wavelengths:
Princeton Optronics makes illuminator chips at different wavelengths. The wavelengths are red (650nm, other wavelengths are custom made), near IR (780, 808, 830, 850, 976 and 1064nm). Now we are offering a 1550nm illuminator product as well.
Low Power Illuminator:
We sell low power illuminators of power levels between 2 to 10W for a number of applications including perimeter security, area illumination border security etc. They are small form factor, high efficiency and low cost.
Fig 19. Picture of a 2 to 10W VCSEL diode laser illuminator (808nm, 976 and 1064nm). The dimensions of the illuminator is 2x2x2".
High Power Illuminators for Military and Industrial Applications:
Using the VCSEL chips Princeton Optronics has developed a number of self contained illuminator modules and in the process of developing other versions of illuminators for military and commercial applications. A 120W battery power portable illuminator is shown in Fig 20.
Fig 20. The picture of a battery powered self contained illuminator module. This has remote trigger as well as blinking lights indicating on/off condition.
Fig 21. The picture of a small 800W illuminator which are used for high speed photography. People are using this illuminator for imaging ultra-fast events with million frames per second camera speed.
A military version of the illuminator has a power output of 650W and has a beam divergence of 20mR This illuminator can be used for imaging through smoke, fog and explosive events. Fig 22 shows the picture of this illuminator. Fig 23 shows a 300W dome illuminator, illuminating at a divergence angle of 120 deg angle for illuminating a large area from above. Fig 20 and 21 shows the illuminated field and a test resolution target imaged through explosion by illuminating it with Princeton Optronics illuminator(Ref M Mentzer et al, Optical Engineering, April 2011 Vol 50(4), 043201-043206)
Fig 22. Picture of an illuminator delivering 650W with a beam divergence of 20mR for illumination of a small target at a long distance. The illumination intensity is 50,000 lumens/ sq meter at a distance of 50 meters. The dimensions of the product is 21x17x19 inch.
Fig 23: Dome shaped 300W area illuminator with 140 deg divergence for illuminating a large area uniformly from above.
Fig 24 shows part of the illuminated area through a CMOS silicon camera at 100W illumination. Fig 25 shows the image through explosions when the scene is illuminated by narrow band high power light from VCSEL arrays and imaged through filter.
Fig 24. Shows part of the area illuminated by the 400W diode laser illuminator module at a power level of 100W.
Fig 25. Imaging of resolution test target through a fireball created by an explosion using a Princeton Optronics illuminator (Ref Dr Mark Mentzer et al, US Army Aberdeen Proving Grounds, Optical Engineering, April 2011 Vol 50(4), 043201-043206)
XI. VCSELs for Pumping:
One major application of VCSELs is for solid state laser pumping applications. Princeton Optronics is making pumps for solid state lasers as well as make some solid state lasers using such pumps. VCSELs are used for end pumping in a configuration shown in Fig 26.
Fig 26. Common configuration for end pumping a Nd:YAG rod. Customers use VCSEL array to obtain 10 to 40mJ Q-Switched power from the laser.
We have pump products of many power levels. One pump laser which is frequently used is shown in Fig 27. The pump power level is >800W as can be seen in the curve below.
Fig 27. A pump laser for end pumping of Nd:YAG rods. Different power levels are used for different applications.
VCSEL Diode Laser Multichip Pump Modules for Side Pumped Solid State Lasers :
By mounting several VCSEL pump arrays on micro-channel cooler or on cold plates, we make pump modules which are used for pumping of side pumped or end pumped solid state lasers. We have several side pumped products. Fig 28 shows a 12 chip module mounted on microchannel cooler. The individual chips are 100W each 808nm in QCW mode. Fig 28 shows the module drawing as well as the picture of module mounted on the heatsink. The modules are stackable side by side so that much higher level of power can be achieved for longer gain materials.
Fig 28. Shows a side pumped Nd:YAG laser for generation of pulsed blue laser of 10mJ/pulse energy. The modules in the picture below are used in generation of such pulses.
Fig 29. Top- drawing of the 808nm chips on submount for side pumping. Bottom- picture of chips on submount and mounted on the heat-sink for actual side pumping application. The module power is 1200W QCW with 10% duty cycle. In CW configuration, the module delivers a power of >400W.
A very high power product is offered with power of >2kW at 808nm is for side pumping of Nd:YAG slices which are cooled from top and bottom.
Fig 30. Multiple 100W 808nm VCSEL array mounted side by side to deliver >2kW of CW power for side pumping of Nd:YAG slice. The chips are mounted in a modular configuration, so that the length can be increased or decreased easily.
XII. High Speed VCSEL Diode Laser Devices:
We are developing high speed VCSEL devices. Our current results are that the devices work to 5GHz speed. Fig 33 shows the performance of a 4u aperture device performing at >5GHz speed. Our devices are much higher power compared to comparable industry standard devices for high speed. We obtain a single mode power level of >5mW at these speed. Princeton Optronics differentiator for high speed VCSELs are the higher power (>5mW) of the single devices and high efficiency of ~45% from such devices.
Figure 31. A 4u aperture single mode device shows high speed performance through >5GHz. The device power is >5mW.
Linear arrays of high speed VCSELs:
We made linear array of high speed VCSELs for some applications. Individually addressed linear arrays have been made for certain applications and we are able to manufacture such arrays in high volume.
XIII. Blue, Green and UV Laser for Projection displays, undersea communications, read/write for optical storage etc.
High Energy Pulsed Blue Laser from Frequency Doubled 946nm Nd:YAG Laser:
Princeton Optronics has developed very high energy blue laser delivering 10mJ/pulse pumped by VCSEL pumps. The beams are single mode and has high efficiency of conversion from IR to blue. The VCSELs are frequency doubled for blue and green and quadrupled for UV. We use PPLN and KTP crystals for frequency doubling and quadrupling.
XIV. VCSEL pumped Solid State Lasers at 1064, 473nm and UV:
Princeton Optronics is making solid state laser modules for high temperature operation. We are making SSL modules at 1064nm operation as well as frequency doubled SSL modules at 473nm. The 1064nm modules are end pumped as shown in Fig 34. The end pumped modules are has a power of 15mJ per pulse with a repetition rate of 20Hz and the 473nm modules have pulse energy of 10mJ with 10ns pulse duration and a repetition rate of several hundred Hz. The picture of a 1064nm module can be seen below in Fig xx. The UV laser module can be seen in the fig below and delivers an energy of 12mJ and average power of 250mW. Princeton Optronics OEMs this product for some applications.
Fig 32. End pumped solid state laser module with output at 1064nm. Pulse energy is 12mJ per pulse and average power is about 250mJ.