Laser Modules for Electro-Optical Pods

An electro-optical pod is a closed system with a fixed mass budget, a fixed power budget, a fixed volume, and a structural loop that must hold boresight across the full operating temperature range. A laser module that meets its datasheet on a bench can still fail in a pod for reasons that have nothing to do with laser performance.

This page covers the six integration constraints that determine whether a module works in a pod: size, weight, power, interface, optical mounting and system integration.

1. Size: the envelope drives the architecture

Pod volume is allocated before component selection, so the module envelope is usually the first filter.

Model	Envelope (mm)	Shape characteristic	Best fit
LDR20K1	≤ 92 × 67 × 53	Flat, wide	Low-profile plate mounting
LDR25K1	68 × 52 × 90	Narrow, tall	Vertical bay beside optics
LDR40K3	95 × 97 × 48	Square, very flat	Sandwiched between decks
LDR40K1	115 × 65 × 60	Long, flat	Longitudinal channel
LDR60K1 / LDR80K1	≤ 116 × 52 × 90	Long, narrow, tall	Narrow side bay
LDR80K2	≤ 150 × 102 × 55	Large, flat	Dedicated flat deck
LR120K1	≤ 75 × 90 × 220	Very long, slender	Longitudinal axis channel

The two modules with the same mass (LDR60K1 and LDR80K1, both ≤ 680 g) share an identical envelope, so a pod designed around one accepts the other without mechanical change. That is a deliberate design decision: it lets an integrator offer two energy levels from one mechanical layout.

Two envelope traps worth checking early:

• Window clearance. The transmit aperture needs unobstructed line of sight; a module that fits the bay but has its aperture partly shadowed by the pod frame will lose energy and distort the beam.
• Connector protrusion. Datasheet envelopes typically exclude the connector. Add 15–25 mm on the connector face unless confirmed otherwise.

2. Weight: the installed number, not the module number

Module mass is only part of the payload mass. A realistic installed-mass multiplier for a pod is 1.5–2.0×:

Item	Typical addition
Module	290–850 g (LDR family)
Mounting bracket + datum plate	10–20% of module mass
Thermal strap / cold plate	10–30% depending on duty cycle
Harness + connectors	5–15%
Window + retaining ring	2–8%

For a 680 g module running designation duty cycles, a 1.2 kg installed mass is a realistic planning figure. Pod budgets that use the bare module mass are usually the ones that come up short in the mass review.

3. Power: three numbers, not one

ERDI long-range modules specify supply 18–32 V DC (28 V typical), average consumption limit 12 W, peak consumption limit 25 W, and power-on inrush current limited to 6 A.

Design your supply for the peak and the inrush, not the average:

• Average in ranging — intermittent, low duty.
• Average in designation — sustained within the qualified duty cycle; this is the thermal design case.
• Peak / inrush — determines bus sizing and whether other pod equipment sees a droop at switch-on.

If the pod bus is 28 V nominal, the module's range covers it directly. If the bus is 12 V or 48 V, a converter is required, and that converter adds mass, volume and its own failure mode.

4. Interface: settle it before the software schedule

RS-422 is the standard command interface, with CAN available depending on model and configuration.

Six things to fix in writing before integration begins:

1. Command set: request range, start designation, stop, read status
2. Response format: binary/text, fixed/variable length, endianness
3. Worst-case latency from command byte to range value
4. Error semantics: what "no echo" looks like versus a timeout
5. Rate: sustained measurements per second
6. Isolation: digital ground, power ground and housing must be electrically isolated

The isolation requirement constrains the mount. An isolated mount is mechanically different from a bonded one, and this is frequently discovered late.

5. Optical mounting: boresight is a structural problem

The laser axis must remain aligned to the EO sensor axis across temperature. At 10 km, a 0.3 mrad misalignment is a 3 m offset — larger than a vehicle.

Three design rules:

• Share one rigid datum. Laser and EO sensor should mount to the same structure, not to two brackets on opposite sides of the pod.
• Keep thermal paths symmetric, or keep them apart. Asymmetric heating bends structures and walks the boresight.
• Design in a collimator check. Boresight verification must be possible on the pod, not only in a workshop.

Note also that divergence and pod stabilisation must match. A 0.15 mrad beam on a pod with 0.3 mrad residual jitter gains nothing from its tightness — the beam walks off the target anyway.

Pod residual jitter	Sensible module divergence
< 0.1 mrad	0.15 mrad (LR120K1, LDR80K2)
0.1–0.3 mrad	0.3 mrad (LDR60K1, LDR80K1)
> 0.3 mrad	0.6 mrad (LDR20K1)

6. System integration: thermal and qualification

Designation is the thermal design case because the laser runs continuously within its cycle. Qualified cycles for long-range modules are specified as on-time and interval per mode; if your concept of operations exceeds them, the thermal design must change, and that is a development conversation rather than a setting.

Module-level environmental qualification is specified, but the installed assembly must be qualified separately: bracket, harness, window and all.

Pod integration checklist

Stage	Verify
Mechanical	Datum flatness, hole pattern, fastener torque, window clearance, connector protrusion
Electrical	Inrush with pod supply, isolation, interface timing under load
Optical	Boresight to EO sensor at both temperature extremes; divergence vs jitter match
Thermal	Base-plate temperature at qualified duty cycle inside the pod enclosure
Software	Latency, error handling on dropped response, self-test semantics
Qualification	Installed-assembly vibration, shock, thermal soak

Frequently asked questions

Which ERDI module suits a small EO pod?

The LDR20K1 at ≤ 290 g in a 92 × 67 × 53 mm flat envelope. For more energy in a similar class, the LDR25K1 at ≤ 380 g in 68 × 52 × 90 mm.

Can one module range and designate?

Yes. Most ERDI 1064 nm modules combine both channels, so the pod carries one module, one mount, one harness and one thermal path.

What power does the module need?

18–32 V DC, 28 V typical, with a 12 W average and 25 W peak limit on long-range models. Design the bus for peak and inrush.

How is boresight maintained over temperature?

Through a shared rigid datum, thermal symmetry and in-service collimator verification. It is a mechanical design problem, not a software correction.

Does the module include a window?

The module is sealed; the pod aperture window is an integration item. Specify coating and transmission with the module supplier before finalising the optical design.

Related: electro-optical system integration, laser rangefinders for UAV gimbals, SWaP trade-offs, interface selection.