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On-chip gratings improve stability of laser diodes

06 Sep 2006

Quintessence Photonics has written gratings into its infrared laser diodes that narrow the emission spectra and reduce temperature sensitivity. As Paul Rudy explains, this makes the diodes ideal for medical applications and could lead to cheaper diode-pumped systems.

The combination of compactness, low running cost and excellent electrical-to-optical efficiency has enabled high-power edge-emitting laser diodes to serve many applications in industrial, medical and defence markets. A growing number of these lasers are directly addressing “thermal” applications such as printing, medical and plastics welding, but the majority have a well-defined emission spectra and are used as sources to pump solid-state and fibre-laser systems.

The advantages of diode pumping over lamp pumping are well known, and include increased system efficiency, greater reliability and lower cost of ownership. However, these systems cannot deliver the temperature-independent performance of lamp-pumped designs because of the laser’s lack of stability. Instead, thermal management and temperature control of the diode are needed to precisely tune its emission wavelength. But even with this control, the linewidths produced are insufficiently narrow for some applications.

It is critical to improve the stability and spectral narrowing of high-power laser diodes so that they can simultaneously deliver the efficiency associated with diode pumping and temperature stability provided by lamp pumping. If these objectives are met at a well-defined wavelength, then laser system design- ers can improve the decvice’s compactness, efficiency, power and beam quality while reducing its thermal-management cost.

The improvements would also mean that these lasers could be used directly for scientific and medical pumping applications, such as Raman spectroscopy and enhanced magnetic resonance imaging, which require precise tuning of narrow emission wavelengths to hit atomic or molecular absorption spectra.

Various methods have already been used to improve the spectral brightness, stability and accuracy of laser diodes. These approaches include various external techniques using either volume Bragg gratings, external lenses and bulk gratings, or seed lasers in master oscillator power amplifiers. However, all of these approaches require sensitive and high-precision alignment, costly additional lasers and/or optics and specially designed coatings. On-chip solutions are possible with internal distributed feedback gratings similar to those that are used in singlemode telecom lasers. However, it is difficult to transfer this technology to high-power multimode lasers because multimode devices require more complex grating designs to capture and lock the large number of transverse modes.

Recently, Quintessence Photonics Corporation (QPC) has overcome these challenges and demonstrated a range of high-power lasers operating at 808, 976, 1470, 1535 and 1550 nm, which are fabricated at our headquarters in Sylmar, CA. These MOCVD-grown InP-based and GaAs-based lasers feature internal gratings that narrow the spectral linewidth, reduce wavelength-temperature sensitivity, and ensure that the device operates at the required wavelength.

High-power laser diodes are usually constructed by inserting a gain-producing active stripe into the device’s resonant Fabry-Pérot cavity. Aside from defining a periodic “comb” of resonant frequencies, the cavity provides no wavelength control. The emission wavelength is controlled by the active layer’s gain spectrum. Unfortunately, this gain spectrum is “flat”, has a characteristic width of typically 20 nm, and is strongly temperature dependent. This makes for a spectrally broad laser output, particularly at high power fluxes, which is highly dependent on the operating temperature. The emission wavelength can typically vary by 0.3 nm/°C.

However, when the on-chip grating is added to select the longitudinal mode, temperature sensitivity is governed by the changes in refractive index of the grating region, and is reduced to 0.1 nm/°C or less.

These devices are fabricated in a similar way to conventional laser diodes, with the gratings defined by optical lithography into a photoresist, followed by etching, or formed during a growth and re-growth process.

The InP and GaAs lasers have different grating geometries that are designed through extensive modelling, but use similar processes to write the gratings. After the design has been optimized, the total processing time for the grating-based lasers is only slightly longer than that for conventional emitters. Our development has led us to believe that high-power grating-based lasers promise excellent manufacturing yields through improved targeting of the wavelength, which leads to reduced yield loss compared with conventional laser diodes.

When 808 nm pump lasers are sold, it is typically with a 3 nm centre wavelength tolerance, a spectral width of less than 2-4 nm and a 0.3 nm/°C temperature tuning coefficient. However, for common gain media, such as neodymium-based crystals, absorption peaks can be as narrow as 1 nm. This means that system manufacturers have to control the operating temperature to within 0.1 °C to correctly tune and maintain the appropriate emission wavelength. Unfortunately, the diode red-shifts as it ages, and to maintain efficient lasing the diode has to be increasingly cooled, often until it reaches the dew point. Once this point is reached, catastrophic damage to the laser’s mirrors can occur.

QPC released 808 nm lasers in June with 100 μm wide stripes that avoid these issues by using internal gratings to deliver the performance described in the table above. These lasers have much narrower laser emission widths than their Fabry-Pérot cousins (see figure 1), and have great promise for Raman spectroscopy, pumping alkali vapours for medical imaging and atomic vapour lasers, and simplifying neodymium-based diode pumped systems.

In the 915-976 nm regime, high-power laser diodes are used to pump fibre lasers that have a typical centre wavelength tolerance of 5 nm, a spectral width of less than 5 nm and a temperature tuning coefficient of 0.3 nm/°C. The fibre laser’s absorption spectrum has a relatively weak broad peak of 915-960 nm, and a peak that is three to four times a strong at 976 nm. Using this shorter wavelength peak is not ideal for a growing number of pulsed fibre laser applications, because longer lengths of fibre increase nonlinear losses. Until now, the choice has been between using an uncooled diode to pump the broad but weak absorption peak, or a temperature-controlled laser to excite the stronger and narrower 976 nm peak. However, our 976 nm single-emitting device shows that it is possible to enjoy the benefits of pumping strong but narrow peaks without the need for high precision temperature controls.

Laser diodes emitting between 1.4 and 1.6 μm are used for various applications, including pumping Er:YAG lasers that are used for range finding, materials processing and aesthetic medical treatments. Er:YAG sources, which emit in the eye-safe regime, are also becoming widely used to reduce the impact of potentially hazardous unintended scattered radiation from either laser sources, optical delivery systems and targets. Applications are plentiful in the industrial, defence and medical markets.

For Er:YAG pumping, lasers operating at 0.9-1.0 μm can be used, but optical conversion is more efficient at 1532 nm where there is a 1 nm-wide absorption peak. This peak can be pumped using typical high-power temperature-controlled InP lasers that have a 10 nm spectral width and 0.35 nm/°C temperature tuning, but it can also be excited with increased efficiency with our grating-based laser bars.

Fibre laser sources High-power fibre lasers often use several expensive amplifying stages, but this could be avoided by using 1550 nm single frequency, single transverse mode diodes that can deliver sufficient power. At higher powers, singlemode operation has been demonstrated in tapered devices. However, producing more power while maintaining a near diffraction-limited performance and narrow linewidth is challenging, because of yield losses owing to beam quality deterioration at high powers, and filamentation at relatively low powers. These issues have been addressed with QPC’s high-power 1550 nm laser, which contains a buried heterostructure singlemode waveguide and a tapered gain region. The waveguide acts as a mode filter, but once the beam is fed into the tapered gain region the mode can freely diffract and be amplified by a tapered electrical contact. These lasers can deliver more than 1.5 W at 28% wall-plug efficiency, using a 5 A drive current. Spectral linewidth is limited by the test equipment, but was measured at less than 6 MHz, and suppression of the sidemodes is more than 50 dB.

The combination of our range of diodes’ spectral brightness, stability and spatial brightness opens the door to deployment in tasks such as the seeding and core pumping of fibre systems, as well as providing the source for second harmonic generation of light for biotech and display applications. And even higher output powers could be reached while maintaining diffraction-limited performance if emitters can be coherently combined. Our motivation is to expand the number of pumping and direct diode applications with enhanced performance, increased temperature stability and reduced system complexity, while maintaining the device’s compactness, low running cost and excellent efficiency.

Acknowledgments

Part of this work was supported by the Naval Air Warfare Center Weapons Division and by the US Army.

• This article originally appeared in the August issue of Compound Semiconductor magazine.

Optikos Corporation LightTrans International GmbHEKSMA OpticsChangchun Jiu Tian  Optoelectric Co.,Ltd.DIAMOND SASynopsys, Optical Solutions GroupHyperion Optics
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