Technology

Princeton Optronics has several key technologies on high power Vertical Cavity Surface Emitting Lasers (VCSELs) and for low noise ultrastable lasers.

VCSEL: Although historically VCSELs have been confined to low-power applications, Princeton Optronics has developed key technologies resulting in the world's highest power single devices and 2-D arrays. We have successfully demonstrated single devices with > 3W CW output power and large 2D arrays with > 230W CW output power. The highest wall plug efficiency of these devices is 52%. Princeton Optronics is a participant in the DARPA-SHEDS program, whose main objective is to improve laser diode conversion efficiency.

We also have developed arrays that can be operated in QCW and pulsed (10-100ns duration) modes and arrays of single mode VCSEL devices which can be coupled to fibers as well as frequency doubled the VCSEL devices and arrays for blue lasers.

VCSELs have several major advantages over edge-emitting lasers:

• A circular output beam. Very seldom are external optics required.
• A much lower wavelength-dependence to temperature (5x less than for edge-emitters).
• A much higher reliability than edge emitters since VCSELs are not subject to COD (catastrophic optical damage) failures.
• High temperature operation. VCSELs and VCSEL arrays can be operated at temperatures up to 80 deg C ambient and can be operated without chillers
• High power from the arrays. We have demonstrated a CW power density of 1200W/cm² from the arrays and expect to go to much higher levels in future
• VCSELs can be easily processed into large 2D arrays (does not need stacking) to scale up the power. The 2-D array configuration provides for more efficient heat-sinking and better pump power density as the devices can be very closely packed.
• Low thermal resistance for CW operation. A thermal resistance of 0.15 deg C/W from the junction temperature to the microcooler coolant makes it lower by over a factor of 2 compared to edge emitter arrays.

For more information on VCSEL technology, please visit the following links:

• Introduction to VCSELs
• High-power CW and QCW VCSEL arrays
• High Brightness VCSEL Array
• Single Mode VCSEL Devices and Fiber Laser Seed Laser
• Blue laser
• VCSEL Reliability

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High Brightness VCSEL Arrays:

Applications: Pumping of solid state lasers, pumping of fiber lasers, illuminators, beacons, narrow divergence high power beams, etc.

VCSELs (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.

Advantages of the VCSELs 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 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 1 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 1)

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Single Mode VCSEL Devices and Application for Seed Laser:

Applications: Wherever small spot size of laser devices with good mode quality and narrow linewidth are needed.

Princeton Optronics makes monolithic single mode devices as well as external cavity single mode devices. Monolithic single mode devices are made at wavelengths of 976nm, and 1064nm. The single mode devices have power of 50mW for extended cavity devices and a power of 5mW for monolithic devices. The single mode lasers with a microlense have a beam divergence of 24mR (1.5 deg FWHM) for monolithic devices and 8 mR (half degrees) for external cavity devices. The beam is difraction limited. The linewidth for both is about <50MHz (measurement accuracy limited). The SMSR of the devices is >40dB.

A very good application for VCSELs is use of them for seed lasers for fiber lasers or other MOPA configurations. The VCSEL has their longitudinal modes very far apart (~40nm) and therefore they do not mode jump from one to the other during modulation. Also, they have high reflectivity output coupler and therefore, they are not damaged by the reflected light. These two properties make them ideally suited for seed laser applications. In addition, they can be modulated at very high speeds of >500MHz.

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Blue Laser:

Applications: Projection displays, undersea communications, read/write for optical storage etc.

Blue Laser From Frequency Doubled VCSEL Devices:

Princeton Optronics has developed very high quality blue lasers. 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 arrays are capable of delivering very high power in 2D array and hence frequency doubled arrays should be able to deliver very high level of power. Princeton Optronics has developed 230W of output power from VCSEL arrays as shown below.

Fig 1. Picture of a 2D array of VCSELs delivering 230W of CW power. The chip has a dimension of 5x5mm and the emission area is 4.7mm diameter.

Fig 2. 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 maximum power of 30mW from a single device has been obtained with an wall plug efficiency of 8%

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Low Noise High Power Ultra-stable Laser:

We have the technology for the world's lowest noise and highest stability laser in the C-band (1550 nm band). For the low noise high power laser, we use an external cavity approach in which we pump a gain medium with a semiconductor laser. The unique cavity design approach has intrinsic advantage of linewidth of 1KHz. and side mode suppression below 70dB. We have a unique technology for RIN noise suppression, we are able to reduce the RIN to near shot noise limit. The noise numbers are -160dB/Hz @1MHz and dropping to shot noise limited regime above 10MHz. There is no RIN peak with this laser because of our RIN elimination technology. We have a novel locker technology (patent pending) to stabilize the lasers to +/- 5kHz level of stability. More details on this technology are available in the following link: Low Noise Laser Technology

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