Coherent
Beam
Combining
(CBC)

Combining laser beams to gain power

One of the architectures implemented for the construction of a high-power fiber laser is the possibility of combining different laser beams. Knowing that today a single fiber laser can deliver a power of the order of a kilowatt, or even ten kilowatts, the optical combination by phasing multiple sources results in a new laser delivering powers of up to several hundred kilowatts. There are several techniques for combining lasers. Among them, two require the use of phase modulators: the spectral combination of laser beams and the coherent combination of beams. For each of these architectures, Exail has managed to develop a dedicated product (bandwidth, driving voltage, suppression of non-linear effects, etc…).

Coherent combination of laser beams

A single laser source with a fine linewidth (on the order of kHz) is amplified several times and feeds an array of phase modulators (a phase modulator is placed on each laser beam path). Phase correction is applied individually for each beam. The modulator dynamically corrects the distortions of the optical paths and maintains the spatial and temporal properties of the output laser beam. In other words, the combination of several lasers by real time control of their relative phases allows to permanently maintain constructive interferences and thus guarantee maximum power efficiency during the combination.

Coherent laser seeder

First stage amplifier

Second stage amplifier

Power combiner

Delivery fiber

Low Frequency Phase Modulator

Transform mirror

Laser output

Coherent laser seeder

First stage amplifier

Second stage amplifier

Power combiner

Delivery fiber

Low Frequency Phase Modulator

Transform mirror

Laser output

Coherent Beam Combination

The phase correction of each beam is carried out in the low frequencies, on the order of kHz, using a DC coupled RF interface. This modulator must be insensitive to temperature variations and not induce any non-linear effects. Excellent polarization extinction ratio is beneficial in such configuration and calls for a polarizing waveguide. Low optical losses are also a benefit with the advantage of withstanding high optical powers to guarantee maximum power from the laser system.

NIR-MPX-LN-0.1 and its matching amplifier DR-VE-0.1-MO are both ideal for high-power coherent beam combining applications. The NIR-MPX-LN-0.1 is also produced using the APE process. The resulting polarizing waveguide can produce a polarization extinction ratio value greater than 60 dB at the output of the optical waveguide chip. The APE process also helps to achieve a high optical input power handling of 300 mW. This modulator also features low insertion loss, and high optical stability.

The modulator is also a DC-coupled device. It means that you can apply a DC voltage to minimize the Residual Amplitude Modulation (RAM) effect (Exail patented technique, US2016109734A1, ”Electro-optical phase modulator and modulation method”). The graph below displays measured RAM values with applied DC voltage. It shows that it is possible, by simply adding a DC component, to reduce Residual Amplitude Modulation by a factor ten or more.

Other components available at Exail for 2 μm applications

The MPX2000-LN-0.1 and its matching amplifier DR-VE-0.1-MO are both ideal for high-power coherent beam combining applications in the 2 μm wavelength band.

Exail’s modulation solution also count the Modbox CBC at 2 µm, an accurate, adjustable, and reliable phase-lock modulation solution for Coherent Beam Combining technique. The ModBox integrates 4 to 8 independent channels with phase and delay tuning capability. Each channel allows an adjustment of the temporal phase for synchronization of all beams. The design integrates Exail proprietary low frequency phase modulator with its matching RF electronic and a tunable optical delay line that is selected for high accuracy and a wide delay adjustment range.

Exail offers a new generation of fibered micro-optics benches integrating within a compact and ultra-stable package a wide variety of optical functions such as power splitters, combiners, optical switches, frequency shifters… As a result, usual free-space optics set-ups are transformed into turn-key ultra-stable integrated sub-systems. These modules feature remarkable stability and are able to operate at high power. For example the Exail Variable Optical Delay Line (VODL) is a device that enables a very precise and stable control of an optical delay up to 12 ns, useful to synchronize laser pulses.

Dedicated products