Technology

WL-XXX FIBER-COUPLED MODULE

Fiber Coupled Module
Fiber Coupled Module

DESIGN STRATEGY

sTRATEGY

The rest of the world (ROW) is focused on commercial applications where cost is the key factor. SLT focuses on military applications where performance is the key factor. 

FIBER-COUPLED MODULE EFFICIENCY

The ex-fiber power-conversion efficiency PCEex-fiber of a fiber-coupled module (FCM) is the product of the laser-diode power-conversion efficiency (PCELD) and the optical coupling-efficiency into the optical fiber (FCE).

PCEex-fiber = PCELD X FCE

Typically, at operating conditions, PCELD is in the range of 60-65%. For PCEex-fiber to be equal to 50%, the FCE must be in the range of 77-83%.

The WL-600 emits 600W from a 225μm, 0.22NA fiber with a PCEex-fiber of 55% at operating conditions. PCELD is ~64% and FCE is ~86%. To increase PCEex-fiber by a percent, PCELD needs to increase ~1.2% or FCE needs to increase ~1.6%.

THERMAL MANAGEMENT: HEATSINK

Laser-diode efficiency is inversely related to laser-diode temperature. Laser diodes with the lowest thermal-resistance (Rth) have the lowest operating temperature and the highest efficiency (PCELD).

Thermal resistance is determined both by the heatsink and the die attach. Water-cooled, copper-based, micro-channel coolers (MCC) have the lowest thermal resistance. Water has the highest heat-transfer coefficient among common liquids. Copper has the lowest thermal conductivity among common metals.

Commercial vendors avoid MCCs since the water quality, e.g. electrical conductivity, pH, and oxygen content, must be carefully controlled for long-life operation.

SLT invented custom MCCs (U.S. Patent #7, 836, 940 B2) for carefree, long-life operation with only distilled water. To meet the storage requirements of Mil-Std 810G, a solution of distilled water and ethanol can be used without any changes to product performance or lifetime.

THERMAL

THERMAL MANAGEMENT: DIE ATTACH

The performance and lifetime of laser diodes are affected by mechanical stress. As such, the die attachment process is a critical one. To reduce stress, commercial vendors attach laser diodes onto submounts that have a coefficient-of-thermal-expansion (CTE) close to that of the semiconductor. The solder is Au80Sn20 eutectic that is stable and easy to use.

The use of a submount improves yield and consequently reduce cost, but it adds significant thermal impedance relative to directly bonding the laser diode onto a copper heatsink.

The CTE of GaAs and copper is ~5.6ppm and ~17ppm, respectively. To minimize mechanical stress, SLT directly attaches a 10mm-wide laser-diode bar onto a copper-based micro-channel cooler using a solid-phase die-attach process. This patented process (U. S. Patent # 8,377,797 B1) is proven to be stable and results in laser diode bars with extremely low smile (<1μm).

dieattach

FIBER COUPLING

Background information:

  • Multiple emitters must be used to achieve high power. The present technology is ~10W per emitter.
  • Single-mode, Yb-doped fiber-amplifiers reach the highest power and efficiency when pumped at the peak absorption wavelength of ~976nm.
  • Laser diode bars are presently ~97% polarized

Fiber-coupling efficiency at high optical-power is improved by

  1. Not using polarization multiplexing
  2. Using high fill-factor laser-diode bars
  3. Minimizing the beam-parameter product (BPP) along the fast-axis (FA) and slow- axis (SA) of the laser-diode bar
  4. Equalizing the beam-parameter product (BPP) along the fast-axis (FA) and slow- axis (SA) of the laser-diode bar

For ex-fiber power in excess of ~300W, commercial vendors make use of polarization (PMUX) or wavelength (λMUX) multiplexing. PMUX reduces the power by ~3% whereas λMUX increases the spectral width, and consequently the pumping efficiency, of the fiber-coupled module.

For Item #3, commercial vendors use simple micro-optics, consisting of relatively low-cost, cylindrical lenses, to minimize the BPP along both the FA and SA. Similar to cameras that incorporate precision compound-lenses to achieve a much better image, SLT uses a LIMO (www.limo.de) beam-transformation system to achieve both Item #3 and #4.

Fill factor

CONNECTIONS

Tool-free connectors are expensive but they pay for themselves in reduced installation time and simple ability to replace a damaged unit in the field.

There are three connections to the WL-XXX fiber coupled modules: 1) the optical fiber, 2) the electrical power and 3) the water coolant. None of the connectors require a tool for installation.

  • Optical fiber connector: Optoskand SMAQ (www.optoskand.se)
  • Electrical connectors: RadlokTM
  • Water connectors: Push-to-connect fittings

Connectors

THE PROOF: WORLD RECORD EFFICIENCY (55%) @ 600W

LIV

WL-600: Power conversion efficiency and mode-stripped, ex-fiber (225μm, 0.22NA) power as a function of operating current.

Spectrum

WL-600: Spectrum at 120A operating current

High power, high brightness fiber lasers and amplifiers