Existing thermal management technologies for diode laser pumps place a significant load on the size, weight and power consumption of High Power Solid State and Fiber Laser systems, thus making current laser systems very large, heavy, and inefficient in many important practical applications. To mitigate this thermal management burden, it is desirable for diode pumps to operate efficiently at high heat sink temperatures. In this work, we have developed a scalable cooling architecture, based on jet-impingement technology with industrial coolant, for efficient cooling of diode laser bars. We have demonstrated 60% electrical-to-optical efficiency from a 9xx nm two-bar laser stack operating with propylene-glycolwater coolant, at 50 °C coolant temperature. To our knowledge, this is the highest efficiency achieved from a diode stack using 50 °C industrial fluid coolant. The output power is greater than 100 W per bar. Stacks with additional laser bars are currently in development, as this cooler architecture is scalable to a 1 kW system. This work will enable compact and robust fiber-coupled diode pump modules for high energy laser applications.
This paper expands on previous work in the field of high power tapered semiconductor amplifiers and integrated master oscillator power amplifier (MOPA) devices. The devices are designed for watt-class power output and single-mode operation for free-space optical communication. This paper reports on improvements to the fabrication of these devices resulting in doubled electrical-to-optical efficiency, improved thermal properties, and improved spectral properties. A newly manufactured device yielded a peak power output of 375 mW continuous-wave (CW) at 3000 mA of current to the power amplifier and 300 mA of current to the master oscillator. This device had a peak power conversion efficiency of 11.6% at 15° C, compared to the previous device, which yielded a peak power conversion efficiency of only 5.0% at 15° C. The new device also exhibited excellent thermal and spectral properties, with minimal redshift up to 3 A CW on the power amplifier. The new device shows great improvement upon the excessive self-heating and resultant redshift of the previous device. Such spectral improvements are desirable for free-space optical communications, as variation in wavelength can degrade signal quality depending on the detectors being used and the medium of propagation.
Single mode tapered semiconductor lasers producing watt-class output powers often suffer from beam quality degradation as drive current increases. The dominant degradation mechanism is believed to be poor gain clamping in the periphery of the optical mode; as the injection current is increased, excess gain in this region eventually leads to parasitic lasing in the amplifier section of the device. However, this effect has not previously been directly observed and other effects such as thermal lensing and gain guiding also likely contribute. Nevertheless, it has been previously shown that by engineering the overlap of the gain profile with the nonuniform optical intensity distribution, performance can be significantly improved. In this work, we report on the direct observation and mapping of the 2D gain profile in a tapered semiconductor laser. InGaAsP-based tapered diode lasers are fabricated with windowed openings on the back (substrate) side of the chip. The devices are soldered junction down for continuous wave operation. An optical microscope is used to observe and map the 2D spontaneous emission profile, and hence gain and carrier density, of the device under operation. The results are compared to a theoretical model to better understand the physical limitations of beam quality degradation in tapered diode lasers.
High-performance photodetectors (HPPDs), with high output power and bandwidth, are needed for RF photonics links. Applications for these HPPDs range from high-power remote antennas, low-duty-cycle RF pulse generation, linear photonic links, high dynamic range optical systems, and radio-over-fiber (ROF). Freedom Photonics is a manufacturer of high-power photodetectors (HPPD) for the 1480 to 1620nm wavelength range, now being offered commercially. In 2016, Freedom has developed a HPPD for similar applications extending into the V-band. The basic device structure used for these photodetectors can achieve over 100-GHz bandwidths with slight variations. This work shows data for RF power and bandwidth performance for various size photodiodes, between 10 μm and 28 μm in diameter. Measurement data will be presented, which were collected at both assembly level and for fully packaged detectors. For detector devices with bandwidth performance over 50 GHz, the generated RF power achieved is expected to be over 15 dBm. This performance is exceptional considering the photodiode is fully integrated into a hermetic package designed for 65 GHz. Improvements in the coplanar waveguide (CPW) transmission line and flip-chip bonding design were integral in achieving the higher saturation at the higher bandwidth performance. Further development is required to achieve a >100 GHz packaged photodetector module.
RF photonic systems place extremely high demands on optical component performance. To achieve this, a low noise, high power optical source, a high power, linear and low Vπ optical modulator, sharp and uniform optical filters and high saturation power photodetectors are required. While some of these individual components exist, they have not, to date, been integrated in any currently existing monolithic or hybrid photonic integration platform. In this paper, recent advances in discrete component performance is presented, including optical sources, modulators and detectors. In addition, options for the integration of these components onto an integrated photonic platform is reviewed.
Proc. SPIE. 9730, Components and Packaging for Laser Systems II
KEYWORDS: Packaging, Cooling systems, High power lasers, Laser applications, Resistance, Solid state lasers, Fiber lasers, Semiconductor lasers, Photonics, Diodes, High power diode lasers, Semiconducting wafers, Laser systems engineering, Liquids, Diode pumped solid state lasers
Existing thermal management technologies for diode laser pumps place a significant load on the size, weight and power consumption of High Power Solid State and Fiber Laser systems, thus making current laser systems very large, heavy, and inefficient in many important practical applications. This problem is being addressed by the team formed by Freedom Photonics and Teledyne Scientific through the development of novel high power laser chip array architectures that can operate with high efficiency when cooled with coolants at temperatures higher than 50 degrees Celsius and also the development of an advanced thermal management system for efficient heat extraction from the laser chip array. This paper will present experimental results for the optical, electrical and thermal characteristics of 980 nm diode laser pump modules operating effectively with liquid coolant at temperatures above 50 degrees Celsius, showing a very small change in performance as the operating temperature increases from 20 to 50 degrees Celsius. These pump modules can achieve output power of many Watts per array lasing element with an operating Wall-Plug-Efficiency (WPE) of >55% at elevated coolant temperatures. The paper will also discuss the technical approach that has enabled this high level of pump module performance and opportunities for further improvement.
High power photodiodes are needed for a range of applications. The high available power conversion efficiency makes these ideal for antenna remoting applications, including high power, low duty-cycle RF pulse generation. The compact footprint and fiber optic input allow densely packed RF aperture arrays with low cross-talk for phased high directionality emitters. Other applications include linear RF photonic links and other high dynamic range optical systems. Freedom Photonics has developed packaged modified uni-traveling carrier (MUTC) photodetectors for high-power applications. Both single and balanced photodetector pairs are mounted on a ceramic carrier, and packaged in a compact module optimized for high power operation. Representative results include greater than 100 mA photocurrent, >100m W generated RF power and >20 GHz bandwidth. In this paper, we evaluate the saturation and bandwidth of these single ended and balanced photodetectors for detector diameter in the 16 μm to 34 μm range. Packaged performance is compared to chip performance. Further new development towards the realization of <100GHz packaged photodetector modules with optimized high power performance is described. Finally, incorporation of these photodetector structures in novel photonic integrated circuits (PICs) for high optical power application areas is outlined.
Practical, large-area, high-power diode pumps for one micron (Nd, Yb) as well as eye-safer wavelengths (Er, Tm, Ho)
are critical to the success of any high energy diode pumped solid state laser. Diode efficiency, brightness, availability
and cost will determine how realizable a fielded high energy diode pumped solid state laser will be. 2-D Vertical-Cavity
Surface-Emitting Laser (VCSEL) arrays are uniquely positioned to meet these requirements because of their unique
properties, such as low divergence circular output beams, reduced wavelength drift with temperature, scalability to large
2-D arrays through low-cost and high-volume semiconductor photolithographic processes, high reliability, no
catastrophic optical damage failure, and radiation and vacuum operation tolerance. Data will be presented on the status
of FLIR-EOC's VCSEL pump arrays. Analysis of the key aspects of electrical, thermal and mechanical design that are
critical to the design of a VCSEL pump array to achieve high power efficient array performance will be presented.
We describe the factors that go into the component choices for a short wavelength (SWIR) imager, which include the
SWIR sensor, the lens, and the illuminator. We have shown the factors for reducing dark current, and shown that we can
achieve well below 1.5 nA/cm2 for 15 μm devices at 7°C. We have mated our InGaAs detector arrays to 640x512
readout integrated integrated circuits (ROICs) to make focal plane arrays (FPAs). In addition, we have fabricated high
definition 1920x1080 FPAs for wide field of view imaging. The resulting FPAs are capable of imaging photon fluxes
with wavelengths between 1 and 1.6 microns at low light levels. The dark current associated with these FPAs is
extremely low, exhibiting a mean dark current density of 0.26 nA/cm2 at 0°C. FLIR has also developed a high definition,
1920x1080, 15 um pitch SWIR sensor. In addition, FLIR has developed laser arrays that provide flat illumination in
scenes that are normally light-starved. The illuminators have 40% wall-plug efficiency and provide low-speckle
illumination, provide artifact-free imagery versus conventional laser illuminators.
The novel concept of K-stabilizing layer is reported for the first time. The coupling coefficient (K) which determines the characteristics of distributed feedback laser diodes (DFB LDs) has been controlled by optimizing the grating depth and layer thicknesses. The coupling coefficient is less dependent on the variations of grating depth and layer thicknesses if an optimized K- stabilizing layer (lower index material like InP) is inserted between the active layer and the guide layer. The controllability of the coupling coefficient has been demonstrated by the standard deviation of the lasing wavelength and the threshold current across a wafer, 0.74 nm and 2.67 mA, respectively.
The fabrication and performance of high-speed and low relative intensity noise (RIN) 1.3 micrometers InGaAsP semi-insulating buried crescent (SIBC) Fabry-Perot (FP) lasers with Zn- doped active layers are reported. These SIBC lasers have a 3-dB modulation bandwidth of 19 GHz for pulsed operation and 16 GHz for cw operation, and a RIN below -150 dB/Hz for biased current at 120 mA. This is the highest modulation bandwidth yet reported for InGaAsP lasers with semi-insulating current blocking layers.
We have investigated, experimentally, the coherent operation of 1-dimensional linear arrays of grating coupled
surface emitting lasers for different laser designs (gain lengths, grating parameters). For laser arrays with shailow
grating teeth and strong inter-element coupling a diffraction limited far field of 0.012 degrees full width half
maximum was obtained from up to 6 coupled lasers extending over a length of 3.5mm.
A novel U-groove distributed feedback (U-DFB) laser structure is reported for the first time. This new U-DFB laser shows threshold current of 34 mA, external total quantum efficiency of 0.4 mW/mA from both facets and side mode suppression ratio of 30 dB.