For applications in manufacturing, remote sensing, medicine, military, and fundamental science, an ideal laser would have high output power and a diffraction-limited beam. The figure-of-merit to describe this property is the brightness, which scales proportional to output power and inverse to the beam quality factor M2.

Schematic diagram of a multi-spectral laser on a Si substrate with the SONOI waveguide platform.

Lasers that are both compact and have high-brightness are challenging to realize. As the size of the laser is reduced, either the output power is decreased or the M2 is increased, primarily due to a combination of thermal effects and high optical intensities. Many applications, such as spectroscopy, infrared countermeasures, free-space communication, and industrial manufacturing, can benefit from a light source emitting at multiple frequencies. A compact and high-brightness laser can then be achieved by spectral beam combining. This photonic integrated circuit (PIC) may expand the spectral bandwidth of currently available optical sources at lower cost, smaller size, reduced vibration sensitivity, and higher brightness.

Over the past decade, advances in heterogeneous lasers on silicon (Si) enable such a multi- frequency and high-brightness laser to be integrated on a single cost-effective substrate. With multiple die bonding, materials exhibiting optical gain at various wavelengths are brought together onto a Si chip and lasers are formed with integrated mirrors. Outputs from each laser can be combined with various stages of wavelength division multiplexing optical elements, as shown in the accompanying figure. Semiconductor optical amplifiers (SOAs) are critical components for many kinds of photonic integrated circuits to increase output power or maintain signal levels as the signal propagates through-out a large number of optical components. SOAs can be integrated on the same platform with the lasers on Si and could be used to increase power following each intra-band combiner.

An ultra-broadband multi-spectral laser on Si can be constructed by employing existing heterogeneous integration technology and building lasers on Si by direct wafer bonding. This fully integrated device is illustrated, showing presently demonstrated spectral bands of lasers on Si operating at 1.3-μm, 1.5-μm, 2.0-μm, and 4.8-μm wavelengths, based on indium phosphide (InP). Active devices have also been integrated with Si3N4 at 1.0-μm (based on gallium arsenide (GaAs)) and 1.5-μm. The spectral bands at 0.4-μm and 3.6-μm wavelengths can be realized with gallium nitride (GaN) and gallium antimonide (GaSb) based materials, respectively.

Spectral beam combining of each laser wavelength to a single output waveguide is achieved in several stages from dense to coarse wavelength division multiplexing (WDM). There is no inherent loss to combining different wavelengths, unlike combining identical lasers, which has inherent 1/N combiner loss. Note that AWGs efficiently combine light from more than two channels both for the dense WDM and for the coarse WDM. Adiabatic couplers can be designed to combine light for coarse WDM with just two inputs. Lasers with wavelengths longer than ~1.1 μm are integrated on Si waveguides, while shorter wavelength lasers are on Si3N4 waveguides. The final ultra-broadband combining stage combines light from the visible (0.76 μm) to the mid-infrared (3.6 μm) with high fundamental mode transmission. Furthermore, simulations show efficient operation in the range of 0.35–6.7 μm, however, this was not verified due to limited availability of optical sources.

To obtain high brightness, every element of this PIC must be efficient. For the lasers, high wall-plug efficiency and output power are critical figures-of-merit. SOAs may also be included to boost the power of each spectral band. The beam combining elements must have low on-chip loss and transmit light to the fundamental modes of each wavelength in a single output waveguide.

This work was done by John Bowers, Alex Spott, and Eric Stanton, University of California, Santa Barbara, for the Office of Naval Research. ONR-0037

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High-Power Broadband Multispectral Source on a Hybrid Silicon Chip

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This article first appeared in the September, 2017 issue of Aerospace & Defense Technology Magazine.

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