|It is well known that the output beam characteristic of a multimode laser diode is inherently non-uniform, due to both spatial and temporal variations of the mode profile [Figure 1], that result from thermal lensing and filamentation . These non-uniformities (‘hot-spots’ and ‘dark-spots’) can lead to deleterious effects for many applications, including solid-state laser pumping , Raman spectroscopy of sensitive materials , laser speckle contrast imaging , and laser illumination . There have been many different methods utilized to homogenize the output power of a multimode laser beam. These methods include the use of mode-mixing in long lengths of multimode optical fibers or custom multi-faceted homogenizers [2,5], refractive beam shapers , and orbital rastering . However, many of these beam tailoring techniques are often complicated, expensive, or require large amounts of real estate.||
Figure 1 – Non-uniform far-field intensity profile of conventional multimode laser showing intensity “hot spots”
Figure 3 – Intensity profile of Innovative Photonics Inc. “Stub laser” utilizing proprietary mode-mixing waveguide.
The “Stub laser,” shown below, incorporates a TEC-cooled, Volume Bragg Grating (VBG) spectrum-stabilized, multimode laser diode. This laser diode, housed in a compact 14-pin butterfly package, has a proprietary mode-mixing waveguide for beam homogenization [Figure 2]. Our proprietary multimode wavelength-stabilized laser features high output power (> 300 mW), narrow spectral bandwidth, and a shaped and homogenized beam profile. The shaping and homogenization evenly spread out the power density and shapes the beam to match the field of view of a camera or spectrometer slit with minimal decrease in coherence length . Designed to meet the most demanding wavelength requirements and replace expensive DFB, DBR, fiber, and external cavity lasers, the multimode spectrum stabilized laser offers superior wavelength stability over time and temperature (0.007 nm/°C), and vibration. Multimode laser diodes are available with a narrower spectral linewidth for FWHM < 0.1 nm (0.07 nm typical) upon request. Devices can be spectrally tailored (638 – 1064 nm) to suit application needs and offer side mode suppression ratios (SMSRs) better than 40 dB (70 dB at some wavelengths - available with additional optional filter).
|This “stub laser" diode package configuration has shown improved performance for many of the applications listed above. For Raman spectroscopy, the small footprint, high power, narrow laser linewidth, high signal to noise ratio, and output beam homogenization make this an ideal source for robust commercial systems. For laser speckle contrast imaging , the narrow spectral linewidth, long coherence length, and output beam homogenization contributed to a 50% increase in global spectral contrast.|
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 R. Chimenti, Laser Focus World, Nov 2020, 35 (2020).
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 A. Laskin et al., Proc. SPIE 8600, 860010 (2013).
 Link to RPMC datasheet of stub laser
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