Technology
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:
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.
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 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 diode laser 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.
It is also important to understand that VCSELs consume less material: in the case of a 3" wafer, a laser manufacturer can build about 15,000 VCSEL devices or approximately 4,000 edge-emitters of similar power levels.
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. Contrary to edge-emitters, VCSELs can only operate in a single longitudinal mode.
2. Wavelength uniformity & spectral width: Growth technology has improved such that VCSEL 3" 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.
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.
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 (edge-emitter 980nm pump chip manufacturers typically only get ~500 chips out of a 2" wafer). On the other hand, yields for VCSEL diode lasers exceed 90% (corresponds to ~5000 high-power chips from a 2" wafer). In fact, because of its planar attributes, VCSEL diode laser manufacturing is identical to standard IC Silicon processing.
9. Scalability: For high-power applications, a key advantage of VCSEL diode lasers is that they can be directly processed into monolithic 2-D arrays, whereas this is not possible for edge-emitters (only 1-D monolithic arrays are possible). In addition, a complex and thermally inefficient mounting scheme is required to mount edge-emitter bars in stacks.
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.
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 will significantly reduce the cost of the high-power module. Currently, cost of the laser bars is the dominant cost for the DPSS lasers.
High 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, VCSELs emit in a circularly symmetric beam with low divergence without the need for additional optics. This has been a tremendous advantage for low-power VCSELs in the telecom and datacom 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
Feedback insensitivity
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.
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.
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 2 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 8.
Figure 8. Shows the set up without chiller using a radiator and a water pump in an arrangement like in a car engine.
Figure 9. 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 9a. Fig 9a. Shows the L-I curves for a QCW array at 808nm at different temperatures. The top curve is at 20 deg C and the bottom one is at 85 deg C. Our set up does not go above 85 deg C. Single device results up to 95 deg C operation has been obtained (see fig 11).
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 100oC (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.
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.
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. 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.
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%.
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 12 shows the performance of a 4u aperture device performaning at >5GHz speed. Our devices are much higher power compared to comparable industry standard devices for high speed. We obtain a power level of >5mW at these speed.
Figure 12. A 4u aperture single mode device shows high speed performance through >5GHz. The device power is >5mW.
High Brightness VCSEL Diode Laser Arrays (Arrays of Single Mode Devices):
Applications: Pumping of solid state lasers, pumping of fiber lasers, illuminators, beacons, narrow divergence high power beams, etc.
VCSEL laser diodes (vertical cavity surface emitting lasers) is a new technology for pumping of solid state lasers, including fiber lasers. Because of their circular beam and excellent optical characteristics, it is possible to make high brightness pumps using an array of closely spaced single mode VCSEL devices. The arrays are temperature stable and operate at high temperatures without chillers. They are lower cost as they do not need expensive operations like cleaving of the wafers. These fiber coupled arrays can be combined for higher output power using fiber combiners and coupled with double core fiber to develop small, high performance fiber lasers working at elevated temperatures without chillers.
For high brightness devices and arrays, Princeton Optronics makes self lasing VCSELs or extended or external cavity VCSELs and couples such arrays into fibers using microlenses as shown in Figure 13.
Advantages of the VCSEL diode lasers for fiber laser pumping applications are the following:
(i) Circular beam output and ease of coupling with fiber-individual VCSEL devices and arrays are known for easier coupling to fibers because of their circular beam and does not need complicated beam shaping optics. It is possible to couple them to fibers with very high efficiency.
(ii) Can be operated at high temperature without refrigeration- can be operated at temperatures to 80 deg C, and are cooled by a water pump and fan/radiator combination similar to the way the automobile engines are cooled. The cooling system becomes very small, rugged and portable with this approach.
(iii) Higher power from the array- VCSELs diode laser arrays are now delivering 230W from a 4.7 x 4.7mm aperture (>1kW/ cm2 )- and by fabricating a large 10x10mm array with the same power density we can obtain >1kW power which can be coupled to a fiber.
(iv) Higher reliability and life expectancy (50x) than edge emitters , resulting from removal of the junction from the emission facet and lower output power-density. This greatly reduces the maintenance needs and increases the life expectancy of solid state and fiber lasers which is dominated by pump life.
(v) Superior linewidth and stability, The typical linewidth from a 200W array is <0.8nm and temperature dependence is 0.06nm/deg C which are both much better compared to the edge emitters- highly beneficial to most solid state laser systems.
(vi) Much lower cost of gallium arsenide VCSEL chips and pump modules, resulting from gallium arsenide IC chip like manufacturing technology and similar simple packaging technique. The VCSELs already replaced the edge emitters in the low power applications for their lower cost, beam quality and high reliability. They can be 10x lower cost compared to the edge emitters as has been the case in the low power applications.
Princeton Optronics has demonstrated a coupling efficiency of 70% using a commercial off the shelf microlens array which was not the most optimum match for their 100W single mode array. The coupling scheme is shown in fig 13 along with a package. With a custom microlens array, our simulation shows that we should get a coupling efficiency of >90%. We developed a laser welded small package (2x1.5x0.5") which is able to handle a heat load of 500W (fig 13).
Blue, Green and UV Laser:
Applications: Projection displays, undersea communications, read/write for optical storage etc.
Blue Laser from Frequency Doubled VCSEL Diode Laser Devices
Princeton Optronics has developed very high quality blue lasers from VCSEL devices. It uses its VCSEL technology and frequency double the VCSEL radiation by using a non-linear material. The blue laser output at 480nm is single mode and highly monochromatic with a beam divergence (half angle) of 8mR.
Advantages of VCSELs for blue radiation:
The VCSEL devices and arrays are capable of delivering very high power in 2D array and hence frequency doubled arrays are able to deliver very high level of power. Princeton Optronics has developed 6W of peak power from a single VCSEL device which was frequency doubled and currently working towards a 10mJ pulse, 1KHz rep rate blue laser in a small form factor. Fig 14 shows the schematic of the approach and fig 15 shows the experimental set up for the blue laser using VCSELs.
UV Laser:
For UV laser, we use an external frequency doubler material BGO to double the frequency of the laser. We are working towards developing several mJ of UV energy per pulse from these devices with KHz rep rate. Fig 16 shows the schematics of the approach. We have obtained 1mJ pulse energy, with a 100Hz repetition rate so far.
Figure 14. Shows the schematic of blue laser generation. An external cavity approach is used in which the PPLN is put inside the cavity. We have developed green laser using this approach as well. A peak power of 6W from a single VCSEL has been obtained with this approach.
Figure 15. Shows the experimental set up for frequency doubling of VCSEL devices. PPLN material is used for frequency doubling. Princeton Optronics is working on frequency doubling of high power arrays. A peak power of 6W from a single device has been obtained.
Figure 16. Shows the schematics of the frequency doubling approach to UV wavelengths. We are working towards developing lasers with several mJ per pulse with 1kHz repetition rate.
Narrow Divergence VCSEL Diode Laser Arrays (as low as 0.5mrad has been achieved):
(as low as 0.5mrad can be achieved):
Applications: Illuminators, Designators, Lidars
The VCSEL arrays with microlens collimation become very narrow divergence light source. A single-mode array (Fig. 17) shows the collimation of the beams from individual VCSELs. Using a microlens aligned with the VCSEL array and held in position by laser-welding the holding frames, we can get a divergence of 60mrad (full angle) for self-lasing arrays and 8mrad full angle for external cavity arrays. Fig 17 shows schematically the collimation architecture using microlenses. Using external lenses as shown in Figure 18 one can achieve very narrow divergence with VCSEL arrays. A divergence of 0.5mrad has been achieved using an expander lens and a focusing lens as shown in Figure 2.
Fig 17. Shows the architecture of collimating the individual beams from the VCSELs in the array by means of a microlens array.
Fig 18. Shows the architecture of collimating the individual beams from the VCSELs and further reducing their divergence by using a beam expander and a focusing lens. A divergence of 0.5mR can be achieved from the entire array with this approach.
VCSEL Diode Laser Illuminators:
Princeton Optronics has developed VCSEL arrays of 3, 6 and 15W 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.
Speckle free Illumination:
Illumination from a VCSEL diode laser array is virtually speckle free. 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.
Military Illuminators :
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 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 19 shows the picture of this illuminator.
Figure 19. 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.
In addition to a 650W illuminator, Princeton Optronics has developed an area illuminator module at 808nm which can be used to illuminate a large area. The module has a power output 400W and can be used to illuminate an area of 1.5x1.5km. Fig 20 and 21 show the diagram of the module as well as the picture of it. . Fig 22 shows part of the illuminated area through a CMOS silicon camera at 100W illumination.
Figures 20 and 21. The diagram of the 400W illuminator module (top). The dimensions of the module is 2.5x1.5x5.5". Bottom is the picture of the module.
Figures 22. Shows part of the area illuminated by the 400W diode laser illuminator module at a power level of 100W.
Commercial VCSEL Diode Laser Illuminator Modules:
In addition to the modules built for military applications, we have built lower power modules for use in commercial illumination. These illuminators are used with silicon CCD or CMOS cameras for illumination for perimeter security, area illumination, border security etc. We have built illuminators for 3W, 8W and 40W output power. Fig 23 and 24 show the diagrams for those illuminators. They are small form factor, high efficiency and low cost.
Figure 23. Picture of a 3W and 8W VCSEL diode laser illuminator (808nm). The dimensions of the illuminator is 2x2x2" and uses 15W input power for 3W output and 40W for 8W output. The beam divergence is 16 deg. 976nm and 1064nm versions would be available in near future.
Figure 24. the diagram of a 40W output illuminator. The input power for the illuminator is 200W. The dimension of the illuminator is 7.5 x 6.5 x 12 in.
VCSEL Diode Laser Multichip Pump Modules for Solid State Lasers :
By mounting several VCSEL pump arrays on micro-channel cooler or on cold plates, we can make pump modules which will be ready for pumping of side pumped or end pumped solid state lasers. We have designed and built several such modules several of which are offered as products for easy use for side and end pumping of solid state lasers. Fig 25 shows a 12 chip module mounted on microchannel cooler. The individual chips are 40W each 808nm in QCW mode. Fig 26 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 25 and 26. 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 480W QCW with 10% duty cycle.
We have made similar mouldes for end pumping of solid state lasers. We made a 2x2 array delivering 400W QCW power from a 1x1cm module. Fig 27 shows the drawing of the module of 400W output power from a 1x1cm area. This can be focused on a 3x3mm area of solid state material for pumping.
Fig 27. Drawing and picture of a 2x2 array (top) and picture (bottom) of 100W chips (400W in total) for end pumping of solid state lasers. The newer versions are expected to deliver 2kW form the same module.
Ultralow Noise Diode Pumped Solid State Laser for the 1550 nm Wavelength Band and VCSEL Diode Laser Based Ultra Low Noise Laser at 1064nm.
1550nm low noise laser (200mW):
Introduction
Princeton Optronics was in the DARPA Phor-Front program (2005-2009) to develop the lowest noise laser at 1550nm in the industry and has developed such a laser. The most important aspect of the low noise laser is its absence of RIN peak. Low noise high power lasers are needed for a number of analog and digital communications systems as well as for sensors and analog signal processing applications. At Princeton Optronics we have developed a low noise, high power diode pumped Er:Yb glass laser technology. A major breakthrough has been achieved in this laser by using a non-linear absorbing material in the cavity. This technique has delivered the most effective noise reduction ever reported in a laser so far. Further optimization of materials and their characteristics will be able to reduce the noise by an additional 50dB more. Current laser output powers up to 100mW are available with RIN of <-140dBc/Hz above 100kHz and -145dBc/Hz @1MHz and higher frequencies and shot noise limited above 100MHz (see fig 30 in this white paper). This is enclosed in a small, 2.4" X 3.25" X 0.5" package. The laser wavelength can be selected over the band 1528-1565nm. . A tunable version of this laser is under development. We have developed a wavelength locker using low finesse ULE glass air spaced etalons for standard laser operation to lock any wavelength. For ultra-stable frequency locking new lockers have been developed that use high finesse etalon and have a separate temperature control from the laser. Both lockers have a laser power monitors for normalization and power control. Laser linewidths of 1.1kHz over 1ms and a frequency control of 250kHz for periods up to 1 hour have been achieved.
Laser Design
Erbium doped phosphate glass permits high co-doping with ytterbium ions that strongly absorb at 976 nm and efficiently transfer their energy to the active erbium material. Therefore co-doping the erbium doped phosphate glass with ytterbium drastically decreases the absorption length at the 976 nm pump wavelength so that small solid-state lasers can be built. Aside from the obvious advantage for packaging a short cavity length results in a large longitudinal mode-spacing (>40 GHz). A single longitudinal mode can be obtained by inserting a low-finesse etalon. By using either an air-spaced etalon with a piezo controlled air gap or a temperature controlled solid etalon consisting of a material with high dn/dT different modes in the 1550 nm telecom wavelength band can be selected. Fine tuning of the lasing wavelength is achieved by controlling the cavity length using a piezoelectric moveable output coupler. Fig 28 shows the optical configuration of the low noise laser.
Figure 28. Laser design optical layout.
The singlemode edge emitter pump laser is collimated and directed into the glass. A slight angle is used to prevent back-reflections de-stabilizing the pump laser chip. The gain medium of high phosphate glass doped with Yb and Er ions is Brewster angle wedged to provide a linearly polarized laser output. Both the tunable etalon, for mode selection, and the output coupler, for frequency tuning, use piezo elements.
Noise Reduction
Due to the energy transfer between the co-dopant and the active material the laser shows a strongly reduced sensitivity to fluctuations in pump power. Hence the RIN spectrum is mainly determined by cavity loss perturbations. The RIN spectrum of Er:Yb lasers is close to shot-noise limited at higher frequencies (>10 MHz) but shows a strong peak at the relaxation oscillating frequency, which is in the 100 kHz to 1 MHz range, depending on the cavity layout and laser power. Without active stabilization of the laser cavity the typical observed RIN peak is approximately -70dB/Hz. We have developed a noise reduction technology that is based on intra-cavity non-linear absorption. Figure 29 shows measured RIN spectra for Er:Yb laser with and without noise reduction. With noise reduction the RIN at the relaxation oscillation frequency is reduced by more than 50dB with no relaxation peak. Fig 30 shows the noise measurement results at NRL.
Figure 29. RIN spectrum of laser with noise reduction (a,), (b) is cavity loss transfer function and noise without the noise reducer (c).
Figure 30. Shows a recent measurement by the Naval Research Laboratory of one of our low noise lasers showing a RIN of -145dBc/Hz at 1 MHz when operated at 95mW output power. The shot noise limit is -168dBc/Hz and the laser reaches this above 200MHz.
Sidemode Suppression
The laser technology uses a high finesse laser cavity and this coupled with the solid-state gain medium gives a very high sidemode suppression ratio. There is a strong mode selector in the laser cavity and the result is very good single frequency characteristics. Fig 31 shows the OSA plot indicating >70dB sidemode suppression. The shape of the laser line is limited by the resolution of the Optical Spectrum Analyzer.
Figure 31. Optical spectrum analyzer chart showing >85dB SMSR
Frequency Stability and Linewidth
The instantaneous linewidth of the laser is ~10Hz or less. The linewidth of the laser is thus primarily governed by mechanical noise. The uncontrolled linewidth is ~1.1kHz over 1ms and this can be reduced by laser frequency control systems. The piezo frequency control systems are limited to a control bandwidth of tens of kHz and this will not significantly reduce the linewidth. There are two developments that we are actively pursuing to reduce the linewidth further. One is to improve the laser packaging to reduce the mechanical noise and increase the piezo tuning bandwith. The second development is to use electro-optic tuning. This has been demonstrated in our research experiments and now is being developed further. Experiments at NIST laboratories using their combination of piezo control and acousto-optic control demonstrated a linewidth of 10Hz using the Princeton Optronics laser. This is shown in Fig 32 and required a control bandwidth of >300kHz in the acousto-optic control.
Figure 32. Frequency Spectrum of laser when locked using NIST piezo and acousto-optic controls. Shows linewidth of 10Hz FWHM
TThe linewidth of the laser is measured by self heterodyning technique and we obtain a linewidth close of 1kHz as seen in fig 33.
Figure 33. The linewidth (1kHz) of the laser as measured by self heterodyne technique. A fiber of 40km of length is used for the measurement.
Wavelength stability:
Using the standard locker a wavelength stability of a few hundred kHz for 8 hrs is achieved. We have developed a high finesse ultralocker to more accurately control the laser wavelength. The current performance is +/-125kHz over a 8 hour period. This is shown in Fig 34.
Figure 34. Wavelength stability test performed at NIST. Show wavelength stability of +/-125kHz over a 8 hour period.
Phase Noise
The line width of the laser is very low, below 1kHz. This translates into low phase noise. The freq noise of the laser is shown in Fig 35 in (Hz/rtHz). The measurement of phase noise in dBc/Hz is shown in Fig 36.
Figure 35. A plot of the frequency noise vs frequency of the laser.
Figure 36. The measurement of the phase noise in dBc/Hz over frequency is shown.
Use of Low Noise Laser for Communication Link Application
We have used these lasers for communication links and obtained very high dynamic range in some early experiments. Fig 37 shows results of a two tone SFDR (spur free dynamic range) measurement. An SFDR of 116dB/Hz2/3 was obtained, which was the limit of the measurement set up.
Figure 37. Fiber Link using Laser and Lithium Niobate Modulator - ~40mW into Receiver. The result is measurement limited - RF Spectrum Analyzer SFDR = 116dB/Hz2/3.
High Power (10W), Low Noise, Narrow Linewidth Laser:
Princeton Optronics has developed high power low noise, narrow linewidth laser at 1550nm using a low noise laser seed and using a fiber amplifier at 1550nm. The schematic of the high power laser can be seen in Fig 38.
Figure 38. Schematic of a 10W output low noise, narrow linewidth laser developed at Princeton Optronics.
The laser has impressive performance with SMSR of >70dB. RIN of ~ -160dB/Hz; linewidth of ~ 1kHz, frequency noise of ~ 10Hz/rt Hz. The laser has an unique capability of being frequency modulated by 5 or 10GHz at a modulation rate of 1-10kHz. Fig 39-42 shows the performance of this laser.
Figure 39. The RIN of the 10W laser with frequency. The red curve shows the RIN of the seed laser. The RIN of the high power laser is about 5dB higher than the seed laser at higher frequency.
Figure 40. Linewidth of the high power low noise laser at 3W output as well as at 9.6W output. The linewidth is compared with the seed laser linewidth which has not changed much through the amplifier.
Figure 41. The frequency noise of the high power laser and comparison with the seed laser. The frequency noise does not change through amplification process.
Figure 42. Frequency modulation of the high power laser. A frequency modulation of 5 to 10GHz can be made at a speed of 1-10kHz.
VCSEL Diode Laser Based 1064nm Low Noise Laser:
VVCSELDiode Laser Device Structure & Fabrication
We have a new approach for low noise laser with extremely low noise using VCSELs. For high-power operation, efficient heat-removal is required and therefore a junction-down, bottom-emitting structure is preferred to improve current injection uniformity in the active region and to reduce the thermal impedance between the active region and the heat-spreader. A schematic of the structure without the heat-spreader is shown in Figure 43.
Figure 43. Schematic of the selectively oxidized, bottom-emitting 1064nm VCSEL diode laser structure.
For current and optical confinement, the selective oxidation process is used to create an aperture near the active region to improve performance. A low-doped GaAs N-type substrate is used to minimize absorption of the output light while providing electrical conductivity for the substrate-side N-contact. The growth is performed in a MOCVD or MBE reactor and starts with an AlGaAs N-type partially reflecting distributed Bragg reflector (DBR). The active region consists of InGaAs quantum wells designed for 1064nm emission and strained-compensated using GaAsP barriers. The active region is followed by a high-reflecting P-type DBR. A high-Aluminum content layer is placed near the first pair of the P-DBR to later form the oxide aperture. The placement and design of the aperture is critical to minimize optical losses and current spreading. Band-gap engineering (including modulation doping) is used to design low-resistivity DBRs with low-absorption losses.
Low noise laser cavity:
The low noise laser cavity s shown in Figure 44. The Optical aspect of the setup consists of VCSEL device and an output coupler. A high-quality Etalon and a Brewster plate in the cavity control the single-wavelength operation and linear polarization, respectively. The optical isolator prevents optical feedback from outside of the cavity to maintain the single-wavelength operation. The beam is then coupled into a PM fiber with a focusing lens for the mode matching.
Figure 44. The low noise laser cavity with an etalon and a Brewster plate can be seen.
Figure 45. Shows the plot of RIN vs frequency of the laser. There is no RIN peak in the output of the laser. The power output of the laser is about 100mW.
Power output from the Laser:
The L-I characteristics of the laser can be seen in fig 46. We can achieve a very high power of ~1W from the laser, but we cannot get commercial small isolators to handle powers >100mW for such lasers. Therefore we are restricted to output power of ~100mW from the laser. However, if anyone is interested in using larger isolators which can handle higher power, we can deliver power levels of 600-700mW from these lasers.
Figure 46. The L-I Characteristics for the VCSEL low noise laser.
Laser Linewidth:
The linewidth of the VCSEL lasers is in the range of 50-200kHz from interferometric measurements (fig 47).
Figure 47. The low noise laser linewidth is between 50-200kHz as can be seen in different measurements of the laser at different current settings
Package Dimension:
The laser is housed in a small package of 89 x 38 x 15 mm as can be seen if fig 48.
Figure 48. The low noise laser package dimensions are 89 x 38 x 15 mm.
1064 and 1550nm Seed Lasers:
CW and pulsed seed lasers are needed to seed the DPSS and fiber lasers. Low noise, narrow linewidth and wavelength stability are important characteristics of the seed lasers. We have developed external cavity VCSEL seed lasers at 1064nm for seed laser and a variant of low noise stable laser for seed laser at 1550nms.
Pulsed and CW Seed Laser Diode at 1064nm:
Several types of the semiconductor seed lasers have been developed for the MOPA fiber laser systems, based on edge-emitting distributed feedback (DFB) lasers and Fabry-Perot lasers. However, those edge-emitting diode lasers have some limitations for the pulsed fiber lasers, such as the slow pulse response and low single mode power, or otherwise incorporating a tapered laser or amplifier portion in the seed laser module to boost its power. Basically, a seed laser in a MOPA fiber laser system affects the fiber laser's spectral line width, noise, pulse parameters, and wavelength tuning, etc., compared to the DBR-type fiber lasers. A high-power seed laser can also eliminate the pre-amplifier stage(s) for simplicity and lower cost.
In VCSELs, single-mode operation is generally possible for small diameter devices, and is typically limited to a few mW of power. To improve the single-mode power of VCSELs, an external-cavity configuration can be used in which a longer distance between the device and the external mirror forces it into a single-mode operation. In this scheme, much larger VCSEL apertures can be used, and therefore much higher single-mode powers can be obtained. Several previous research using either optical injection (pumping) or direct electrical injection have successfully demonstrated this approach. There are many advantages to single mode VCSELs which include low-cost manufacturing and high reliability. Our high-power and low-noise seed laser at 1064nm is based on our newly developed large-aperture, and high-power VCSEL devices with nearly-diffraction-limited beam quality and low RIN, as well as fast pulse response, which is ideal for this high performance seed laser applications. Besides fiber amplifiers, this seed laser can also be used in many other applications, such as spectroscopy, and sensing, etc.
Seed Laser Cavity:
The cavity configuration of a VCSEL seed laser can be seen in fig 49.
Figure 49. Shows the cavity configuration of a VCSEL seed laser. It has an etalon and a Brewster plate in the cavity for wavelength selection and polarization.
For pulsed operation, fig 51 shows the optical and electrical pulses .
Figure 50. Shows the LI curve of a VCSEL pulsed seed diode laser. A peak power of 18W has been obtained. More recently, we have obtained peak power of >30W from the laser. Fiber coupled power output of >15W has been obtained.
Figure 51. Electrical and optical pulse shape traces on an oscilloscope. Ch 1 is the Electrical pulse from the pulse driver's current monitor port, and the Ch 2 is the optical pulse measured at the laser's output by a fast Silicon detector.
Fig 52 shows the L-I characteristics of a CW seed laser. A power output of 1W has been obtained with good M2. A fiber coupled power output of 600mW has been obtained out of such devices.
Figure 52. Shows the L-I characteristics of a VCSEL CW seed diode laser.
Seed Laser Diode Package:
The laser is housed in a small package of 89 x 38 x 15 mm as can be seen if fig 53.
Figure 53. The seed laser package dimensions are 89 x 38 x 15 mm.
11550nm Seed Laser:
Our 1550nm seed laser technology is the same as our low noise laser technology and a description of it can be found in the 1550nm low noise laser technology description. We make 50, 100, 150 and 200mW seed laser at 1550nm.
