Photodiodes

Brock Koren

These photodetectors are ubiquitous components in today’s arsenal, guiding bombs and missiles with pinpoint accuracy.

A decade after the Gulf War, the US is again involved in a military campaign, with “smart” weapons playing a key role. In fact, the political success of this effort has depended in part on the ability to keep collateral damage to an absolute minimum while effectively targeting specific groups and installations. Thus, it is easy to argue that smart weapons have never been more important.

Most smart weapons rely on some type of photonics technology to provide target guidance or acquisition. Photodiodes are the photodetectors of choice in many of these weapons because they offer numerous important advantages, such as compact, lightweight packaging, custom configurations, rugged reliability and solid-state efficiency. There are several diverse ways in which photodiodes are used to enable smart weapons.

Laser guidance

The goal of an offensive smart weapon is to actively guide a bomb or a missile to its designated target. Because all onboard systems are destroyed on impact, it is not economical to incorporate complex, expensive components and systems in general-purpose bombs and smaller missiles. A popular approach for these types of ordnance is laser guidance. In this technique, a laser beam marks the surface of the target with an intense spot of radiation. An Nd:YAG at 1.06 μm typically does the marking. This wavelength offers the advantage of stealth (it is invisible) while being easily detectable with silicon PIN photodiodes.

In operation, a laser beam is directed at the target either from the launch platform or by a third party, including covert ground operations personnel. Light reflected from the target enters the front of the missile through the so-called dome. In most cases, the dome optics are molded polycarbonate plastic, to minimize overall cost and weight. These optics also include a bandpass filter to block light outside the laser wavelength. In this way, a high-contrast image of the laser spot is focused onto a photodetector assembly, mounted immediately behind the dome.

Typically, this photodetector is a quad cell — a photodiode whose active area is partitioned into four discrete quadrants. The outputs from these four quadrants are independently amplified by four high-gain transimpedance amplifiers. These are often hybridized (integrated into the photodiode assembly) to maximize signal-to-noise and, hence, range. The ratio of the signals from the four amplifiers enables easy determination of the position of the image spot on the quad cell. Of course, the molded optics may introduce aberrations and may produce a distorted spot on the photodiode array, but these effects are easily calibrated out during missile assembly and test.

The launch platform is equipped with a similar system to allow it to track and observe the target before and after destruction. Here, the photodiode quadrant is usually mounted on a pair of orthogonal gimbal mounts, which provide Θ_X and Θ_Y adjustment/motion, allowing the platform to “look” at targets at wider angles. Information from this detector allows the bomb or missile to be automatically launched in the right direction.

This type of system is referred to as “fire and forget”; after launch, signals from the onboard quad detector in the missile are used to adjust directional vanes on the projectile during its flight. No further direct communication occurs between the launch platform and the missile. Today the military uses these systems to guide both passive projectiles and actively propelled missiles.

In a typical battlefield situation, several targets may be simultaneously designated and marked with separate lasers. To avoid confusion and crosstalk, each laser beam emits a coded sequence of pulses. Each missile is programmed to respond to only one of the code sequences.

Wire guidance

Another type of ordnance that relies on photodiodes is the wire-guided missile — e.g., the tube-launched, optically tracked weapon (Figure 1). The missile remains in direct communication with the launch platform via a long wire that uncoils during flight. This is not a “fire and forget” system; the launch platform — which may be a Bradley vehicle, a helicopter, a Hummer or even a simple portable tripod — tracks the projectile and supplies corrective trajectory information. The idea is to incorporate only the less expensive guidance components in the missile itself, with most of the “smarts” in the launch platform, thus allowing repeated use and lower event cost. Unlike laser guidance, there is no third-party target designator.

Figure 1. In wire-guided missiles, such as the tube-launched optically tracked weapon, most of the “smarts” are incorporated in the launch platform to minimize cost and weight. Courtesy of US Department of Defense.

A bomb-site/telescope arrangement acquires it; the operator sights the target (usually a tank) in the crosshairs of a telescope. The system computer registers the direction of the target from angular encoders on the telescope gimbals. The missile is launched through a tube, which can be at a large (45°) angle from the telescope, depending on the target’s location. The computer immediately corrects the trajectory to direct the missile toward the target.

The system tracks the entire flight by following a xenon lamp in the tail of the missile. The tracking system contains two subsystems to provide wide-field (lower resolution) and narrow-field (high resolution) tracking and correction. These subsystems are similar except for f number and detector size. In each subsystem, the xenon lamp is imaged through a rotating prism onto a two-element photodiode assembly with a characteristic chevron shape (Figure 2). The optics form a separate image on each of the photodiode “legs.” As the prism rotates, this image moves across the detector, producing a modulated signal. This arrangement is used instead of a quad cell because the optics are designed so that the two legs of the photodetector produce orthogonal information on the trajectory angle, which can be directly converted into pitch and yaw corrective commands for the missile.

Figure 2. In the tube-launched, optically wire-guided missile, a chevron-shaped photodiode assembly provides corrective pitch and yaw data.

The missile is also directed to fly a smart trajectory. A standard, “time of flight” pulsed laser rangefinder is mounted inside the launch platform to automatically detect the range. (This also uses a photodiode — in this case, a high-gain avalanche photodiode.) As the missile nears the target (usually a tank), it makes an “up and over” detour so that it can strike from above, effecting maximum damage.

Antimissile weapons

Smart weapons also protect the launch platform by countering incoming missiles. A large, slow-moving ship presents a particularly inviting target. Until recently, missiles such as the Exocet could strike a ship with devastating effect, as evidenced in the Falklands/Malvinas war between the UK and Argentina in 1982. These missiles initially proved hard to combat because of their low trajectory and high speed (up to Mach 2). The task is to accurately direct a defensive missile at the incoming missile and to detonate it in proximity.

A successful solution has proved to be the rolling airframe missile, which, for security reasons, cannot be discussed in anything but general terms here (Figure 3). In this system, photodiodes are used to sense the proximity of the incoming missile, rather than for guidance.

Figure 3. The new ship-based rolling airframe missile system utilizes photodiodes to determine proximity to incoming enemy missiles. Courtesy of Raytheon Co.

This missile is a “fire and forget” system; once launched, it relies on internal tracking systems. Initially, it tracks the incoming missile via the radio frequency signal it uses to lock onto the ship. At closer range, it tracks the infrared from the plume of the incoming missile.

As its name suggests, this missile rolls (spins) during flight at a high revolution rate. Laser diodes are arranged to emit from its side. When the rolling airframe missile is passing alongside the incoming missile, the laser radiation is reflected and collected through optics and a bandpass filter mounted in its side, and then directed onto elaborate arrays of photodiodes. The reflection signal is used as a proximity fuse to detonate the warhead. The success of this missile is due to its ability to discriminate among reflections from the target missile, sunlight, starlight or the ocean surface.

Figure 4. In the latest arc fault detector for the US Navy, a photodiode detects high-voltage arcs by their UV emission. Integrated LEDs allow complete system test.

Laser diode sources and monochromatic (bandpass filter) detection filter out ambient light. The shape of the photodiode array and the characteristic profile of the pulsed signal that results from the high-speed rotation of the missile protect against ocean reflections.

Because most of the smarts are incorporated within this sophisticated weapon, the rolling airframe missile represents a higher cost per launch. But in this case, cost is really not an issue, given the weapon’s importance and the fact that the manufacturer, Raytheon in Tucson, Ariz., reports a kill rate higher than 99.99 percent.

Photonics technology, and the photodiode in particular, has been key in enabling smart weapons that can attack selected targets with great accuracy, avoiding blanket destruction. Now, more than ever, this type of weaponry is critical to the military and political success of warfare.

Meet the author

Brock Koren is president and CEO of Advanced Photonix Inc. in Camarillo, Calif.

Arc Fault Detection Systems

Photodiodes play a role in protecting the launch platform from internal failure problems. For example, ships use high-voltage systems that can arc and create fires. If a fire is not promptly detected and extinguished, it represents a serious risk to personnel — either directly or by detonating stored ordnance. Because of this risk, the US Navy recently developed an automated arc fault detection system that relies on silicon photodiodes from Advanced Photonix Inc.

An electrical arc breakdown generates highly ionized air molecules that emit UV and visible radiation. The arc fault detector gathers this light through an input lens and passes it through a UV bandpass filter. A single-element photodiode that is hybridized with a transimpedance amplifier detects the UV. As soon as an electrical arc occurs, it triggers this device, and the signal is used to automatically shut down the local electrical system before a fire can start. The UV filter eliminates false alarms caused by changes in ambient light.

Because of its importance to the ship’s safety, the system is designed to self-test. Several LEDs are incorporated within the photodiode hybrid assembly and are directed at the front window of the detector (Figure 4). During test, light from the LEDs bounces off the inner surface of this window onto the photodiode, which allows a full system test, including automatic electrical shutdown. Visible LEDs can be used because they are inside the photodiode assembly; their light never passes through the UV bandpass filter.