Safe laser–beam propagation for interplanetary links

Ground-to-space laser links serve both as a beacon and an uplink command channel for deep-space probes and Earth-orbiting satellites.¹ An acquisition and tracking-point design to support a high-bandwidth downlink from a 20cm optical terminal on an orbiting Mars spacecraft typically calls for 2.5kW of 1030nm uplink optical power in 40 microradian divergent beams.² The nominal ocular hazard distance of the uplink exceeds 2E5km, approximately half the distance to the moon. Recognizing the possible threat of high-power laser uplinks to the flying public and to sensitive Earth-orbiting satellites, the Jet Propulsion Laboratory (JPL) developed the three-tiered system at its Optical Communications Telescope Laboratory (OCTL) shown in Figure 1, to ensure safe laser-beam propagation through navigational and near-Earth space.

Figure 1. Jet Propulsion Laboratory (JPL) three-tiered laser safety system at the Optical Communications Telescope Laboratory (OCTL).

The philosophy behind the development of the three-tiered laser safety system (see Figure 2) was to have a self-contained system at the OCTL, where the system (see Figure 3) was installed in 2004.^3,4 Sensors in the first two tiers detect objects in navigable air space. Tier 1 sensors consist of a wide and narrow field long wavelength IR camera bore sighted with the telescope.⁵ The cameras use a Raytheon Control IR 2000B barium strontium titanate focal-plane array. Each array has a noise equivalent temperature difference of 130mKelvin and comprises 320×240 active pixels. The narrow-field camera uses an F/1.0 75mm germanium lens to generate a 12×9 degree field of view with heightened sensitivity to see small aircraft at the full 3.4km design range. The wide-field camera uses an F/1.0 18mm germanium lens for a 46×35 degree field to identify rapid, low-flying aircraft at high angles before they encounter the beam. We aimed both cameras at the same point in the far field. The system is mounted to the telescope structure and bore sighted to the center of the telescope's field of view. The Tier 2 sensor (see Figure 2) is a phased-array, x-band radar capable of detecting a small, single-engine aircraft at a range of 26km and a large jumbo jet 82km away.

Figure 2. Aircraft detection sensors integrated and co-aligned with the OCTL telescope. LWIR: Long wavelength IR.

Figure 3. JPL OCTL facility in Wrightwood, CA.

The Tier 3 region lies within the purview of the US Space Command and provides for safe laser-beam transmission through the near-Earth space environment. JPL registers the OCTL lasers used for ranging and/or optical communications with the US Space Command. JPL also gives the command permission to illuminate the targeted satellite and notifies it of planned laser transmissions to satellites or to a star for alignment calibration. The agency provides the OCTL with a listing of predictive avoidance times when laser transmission is disallowed for any OCTL laser. Figure 4 shows the percentage of time when laser transmission to the star Sirius was precluded for a three-month period. Not all lasers are subjected to predictive avoidance control. Depending on the laser power, wavelength, beam divergence, and operating conditions, some laser links are waived from the procedures. For example, the Tier 3 system control is not invoked for these laser transmissions.

Figure 4. Tier 3 predictive avoidance outages over three months using the star Sirius for laser-pointing calibration.

The sensors from the three tiers are coupled to the laser safety control computer (LSCC) through the laser safety electronics box (see Figure 5). The radar output is also displayed on a monitor. The LSCC monitors and records the inputs from Tier 1, 2 and 3 sensors and from the optical detector. It determines whether all tiers are clear for transmission and releases the beam interrupt shutter, whose default position is closed. The optical detector looks at the laser output partially reflected from a mirror surface to confirm the state of laser transmission.

Figure 5. The laser safety electronics box input/output (I/O) board conditions the sensor outputs for input to the control computer. LCH/PA: Laser Clearing House/predictive avoidance.

The monitor and control computer displays a running graphical representation of the current system status to a five-minute history (see Figure 6). The LSCC data is stored as a record and for future statistical analysis. Since its inauguration in 2003, the LSCC has supported laser-beam transmission into space without incident.^6,7

Figure 6. Laser safety control computer status during operation.

Currently, the Federal Aviation Administration (FAA) requires aircraft observers to be outside the facility during visible and invisible laser transmissions. The laser safety system obviates the need for, and the cost associated with, observers. Future plans call for JPL to continue to work with the FAA to validate the autonomous laser system approach as a safe alternative to posting observers outside laser-transmitting facilities.

This work was performed at JPL, California Institute of Technology, under contract with NASA.