Jonathan M. Saint Clair

Application of COTS high-speed 980-nm pump laser diode and driver for free-space laser communication terminal

In our Photonics West 98 paper, we presented our study results on using commercially available 860 nm high power laser diodes and high-speed laser driver for free-space laser communication terminal application. We demonstrated the feasibility of a free space laser communication link using a junction-up 860 nm high power laser diode driven by a high current laser driver from Hytek Microsystems up to 622 Mb/s. Recent development in high speed InGaAs/GaAs strained layer quantum well (SLQW) laser at 980 nm has provided an additional design option for a laser communication terminal. The advantages of using the 980 nm laser are: (1) WDM market in the telecom industry has created a volume demand for the 980 nm pump lasers. The future cost of 980 nm lasers is expected to be lower due to the economy of scale. (2) In our previous publications, we have demonstrated CW operation of strained layer QW laser at temperature higher than 200 degree(s)C. There is a potential for this type of laser diode to operate in a much harsher and higher temperature environment, and (3) 980 nm pump laser has output power comparable to high power 860 nm laser diodes. In this paper, we will present the high data rate characteristics of a high-speed 980 nm (SLQW) pump laser. Using commercial-off-the-shelf laser drivers we will demonstrate the laser transmitter system characteristics from 622 Mb/s to 3 Gb/s. Detail experimental results on bit- error-rate measurement for a 980 nm device will be presented.

Application of COTS high-power laser diodes and driver for a free-space laser communication terminal

Recently, there has been strong interest in the application of commercial-off-the-shelf (COTS) electronic and optoelectronic (O/E) components for free space laser communication application. Besides the space qualified packaging issues, the main problems of using COTS O/E transmitter are: (1) Telecom grade laser transmitters do not have sufficient power to meet the free space laser communication requirements; (2) COTS laser diode transmitter driver circuits have limited peak drive current, usually below 100 mA, which is too low for driving high power laser diodes; and (3) COTS high power laser diodes are usually not used for high data rate applications since the high speed performance of the laser/driver combination is usually inadequate. In this paper we will present our latest study results on the SDL 5430 and SDL 580 high power laser diodes driven by high current laser drivers at data rates from 600 Mb/s to over 1 Gb/s. Several models from the HY6000 family of high current and high speed laser diode drivers from Hytek Microsystems Inc., designed for free space laser communication applications, have been tested with the SDL high power laser diodes. Using direct drive technique with NRZ modulation, average output power over 100 mW at 622 Mb/s were obtained with these low cost Hytek drivers. For data rates over 1 Gb/s, the parasitic associated with the laser diode becomes an important limitation factor. We have measured the capacitance of the SDL 5430 and the new junction up SDL 580 laser diodes, an equivalent circuit model is developed to examine the effect of these parasitics on the speed of the laser diode. The results are consistent with our experimental observations.

Optical communications receiver array

One of the major challenges to free space laser communications and ladar is the impact of turbulence on beam propagation, one example of which is signal fading. These impacts can be exacerbated on airborne platforms by turbulence in the vicinity of the laser system aperture and the platform wake. There are a number of strategies to mitigate this, including adaptive optics, active flow control, and various dimensions of diversity: wavelength, polarization, temporal, and spatial diversity. In this paper we will discuss spatial diversity implemented in the focal region of optical telescopes. We will briefly compare this with other methods, describe results of requirements analysis of array features and optical configurations for various atmospheric turbulence states, and suggest several attractive configurations. We will also report on the design and test of one configuration, implemented in a prototype, and tested for noise performance, optical transmission, modulation bandwidth, and BER performance with our dynamic turbulence simulator. Early evidence shows significant BER improvements of several orders of magnitude at high turbulence fluctuation frequencies using this technique.

Bi-directional free space laser communication of gigabit ethernet telemetry data using dual atmospheric effect mitigation approach

This paper presents experimental demonstration of optical components applicable in free space laser communication systems for bi-directional transmission of Gigabit Ethernet (GBE) telemetry data and control messages using a dual atmospheric effect mitigation approach. The objective is to address the challenges for optical transmission of telemetry data. (1) Turbulence effects which cause optical beam scintillation, wander and breakup, all of which cause signal degradation at the receiver. (2) An optical signal in free space has a fading effect which is caused by communications terminal equipment‘s in-ability to maintain perfect pointing along a line of sight due to vibrations/motions of the mobile platform.

A tabletop turbulence generator

In order to iteratively improve an atmospheric optical communications system, a benchtop method of testing its behavior must be developed. We have developed a simple table top source of turbulence and instrumented it to accurately measure the current turbulent state. With this tool, the development of optical communication subsystems such as the Boeing Optical Communications Receiver Array (OCRA) can be optimized in the laboratory before they are tested in the atmosphere. The Tabletop Turbulence Generator is uses a small, low speed wind tunnel with a grid of heated rods at the head of the test section. This creates a turbulent flow with a characteristic scale of ~6-30 mm and maximum angular disturbances of ~.1 milliradians. This is a reasonable scale for 1 and 2 inch optical systems. A crucial element in this Turbulence Generator is instrumentation to allow us to measure the turbulent state. We have implemented a Shack-Hartmann sensor operating at 30 Hz and a Malley probe providing time resolved data along two lines of sight with a time resolution of .1 milliseconds. We have used this system to characterize the performance of communications receivers and will present this as a pilot study of the Turbulence Generators performance.