High power 808nm semiconductor lasers are widely used for pumping neodymium-doped yttrium aluminum garnet (Nd:YAG) crystal to produce high-brightness lasing at 1064nm. In addition, there are growing interest to use such high power 808nm lasers in the field of automotive infra-red (IR) illumination and medical aesthetic treatment. Vertical-cavity surface-emitting lasers (VCSELs) have emerged as a promising candidate and attracted increased interests for those applications, due to their combined advantages of high efficiency, low diverging circular beam, narrow emission spectrum with reduced temperature sensitivity, low-cost manufacturability, simpler coupling optics, and increased reliability, especially at high temperatures. They can emit very high power with very high power density as they can be conveniently configured into large two-dimensional arrays and modules of arrays. We report recent development on such high-power, high-efficiency 808nm VCSELs with industrial leading ~55% power conversion efficiency (PCE). Top emitting VCSELs were grown by MOCVD and processed into single devices and 2D arrays using selective wet oxidation process and substrate removal technique for efficient current confinement and heat removal. Peak PCE of 51% and peak power of 800W were achieved from 5x5mm array, corresponding to peak power density of ~4kW/cm2. Pumped with new generation of 2.3kW VCSEL module, Q-switched laser pulse energy at 1064nm reached 46.9mJ, more than doubled from previously reported results.
A compact passively Q-switched Nd:YAG laser was end-pumped by a water-cooled 808 nm vertical-cavity surface-emitting laser (VCSEL) pump module comprising four high power, high brightness VCSEL chips with a combined 10 mm diameter circular emitting area and 2.3 kW total peak power, resulting in 47 mJ laser pulse energy at 1064 nm with 16% optical efficiency at 15 Hz repetition frequency. A laser package comprising an air-cooled 1.6 kW VCSEL pump module produced 37 mJ laser pulse energy, while more than 13 mJ laser pulse energy was demonstrated in a bench-top experiment with a very compact laser set-up using a single 5 mm x 5 mm VCSEL chip.
High power 808 nm vertical-cavity surface-emitting laser (VCSEL) arrays were used to end-pump diffusion-bonded composite laser rods consisting of an Nd:YAG gain medium and a Cr:YAG saturable absorber. The laser pulse energy, q-switch delay time, and optical efficiency of a passively Q-switched monolithic solid state laser in a compact rugged package were measured as a function of VCSEL power for various heatsink temperatures. Up to 19 mJ laser pulse energy was produced with 13% optical efficiency.
Proc. SPIE. 9766, Vertical-Cavity Surface-Emitting Lasers XX
KEYWORDS: Vertical cavity surface emitting lasers, Mobile devices, Gesture recognition, Three dimensional sensing, 3D surface sensing, 3D image capture, Optical filters, Laser sources, Structured light, High power diode lasers, Time of flight imaging, Diffusers, Temperature metrology, Reliability, Infrared imaging, Stereoscopy
There has been increased interest in vertical-cavity surface-emitting lasers (VCSELs) for illumination and sensing in the consumer market, especially for 3D sensing ("gesture recognition") and 3D image capture. For these applications, the typical wavelength range of interest is 830~950nm and power levels vary from a few milli-Watts to several Watts. The devices are operated in short pulse mode (a few nano-seconds) with fast rise and fall times for time-of-flight applications (ToF), or in CW/quasi-CW for structured light applications. In VCSELs, the narrow spectrum and its low temperature dependence allows the use of narrower filters and therefore better signal-to-noise performance, especially for outdoor applications. In portable devices (mobile devices, wearable devices, laptops etc.) the size of the illumination module (VCSEL and optics) is a primary consideration. VCSELs offer a unique benefit compared to other laser sources in that they are "surface-mountable" and can be easily integrated along with other electronics components on a printed circuit board (PCB). A critical concern is the power-conversion efficiency (PCE) of the illumination source operating at high temperatures (>50 deg C). We report on various VCSEL based devices and diffuser-integrated modules with high efficiency at high temperatures. Over 40% PCE was achieved in broad temperature range of 0-70 °C for either low power single devices or high power VCSEL arrays, with sub- nano-second rise and fall time. These high power VCSEL arrays show excellent reliability, with extracted mean-time-to-failure (MTTF) of over 500 years at 60 °C ambient temperature and 8W peak output.
We are developing VCSEL technology producing >100mW in single frequency at wavelengths 780nm, 795nm and 850nm. Small aperture VCSELs with few mW output have found major applications in atomic clock experiments. Using an external cavity three-mirror configuration we have been able to operate larger aperture VCSELs and obtain >70mW power in single frequency operation.
The VCSEL has been mounted in a fiber pigtailed package with the external mirror mounted on a shear piezo. The package incorporates a miniature Rb cell locker to lock the VCSEL wavelength. This VCSEL operates in single frequency and is tuned by a combination of piezo actuator, temperature and current. Mode-hop free tuning over >30GHz frequency span is obtained. The VCSEL has been locked to the Rb D2 line and feedback control used to obtain line-widths of <100kHz.
We have achieved a 21.2% wall-plug efficiency green laser at 532 nm based on an electrically pumped vertical externalcavity surface emitting laser (VECSEL) through intracavity second harmonic generation. The continuous-wave green output power was 3.34 W. The VECSEL gain device is cooled by using a thermoelectric cooler, which can greatly benefit packaging. Both power and efficiency can be further scaled up by optimizing external-cavity design and improving the performance of VECSEL gain device.
Vertical-cavity surface-emitting lasers (VCSELs) are attractive for many pumping and direct-diode applications due to combined advantages in low cost, high reliability, narrow and thermally stable spectrum, high power scalability, and easy system integration, etc. We report our progress on electrically pumped, GaAs-based, high- power high-brightness VCSELs and 2D arrays in the infrared wavelength range. At 976nm, over 5.5W peak CW output and 60% peak power conversion efficiency (PCE) were demonstrated with 225um oxide-confined device. For 5x5mm arrays, peak PCE of 54% and peak power of >450W at 976nm, peak PCE of 46% and peak power of >110W at 808nm were achieved respectively under QCW conditions. External cavity configuration was used to improve the VCSEL brightness. Single mode output of 280mW and 37% PCE were realized from 80um device. For large 325um device, we obtained single mode (M2=1.1) CW output of 2.1W, corresponding to a brightness of 160MW/cm2*sr. Three major areas of applications using such VCSELs are discussed: 1. High brightness fiber output; 2. High power, high efficiency green lasers from 2nd harmonic generation. 3.34W green output with 21.2% PCE were achieved; 3. Pumping solid state lasers for high energy pulse generation. We have demonstrated Q-switched pulses with 16.1mJ at 1064nm and 4.9mJ with 1W average power at 473nm.
We report on a Q-switched VCSEL side-pumped 946 nm Nd:YAG laser that produces high average power blue light with high pulse energy after frequency doubling in BBO. The gain medium was water cooled and symmetrically pumped by three 1 kW 808 nm VCSEL pump modules. More than 1 W blue output was achieved at 210 Hz with 4.9 mJ pulse energy and at 340 Hz with 3.2 mJ pulse energy, with 42% and 36% second harmonic conversion efficiency respectively. Higher pulse energy was obtained at lower repetition frequencies, up to 9.3 mJ at 70 Hz with 52% conversion efficiency.
For infrared illumination with wavelength range of 808nm-1064nm, vertical-cavity surface-emitting lasers (VCSELs) offer many advantageous properties including superior beam quality (such as low divergence, circular shape beam and speckle-free image), increased eye safety, high reliability and low manufacturing cost. We report our progress on highpower high-efficiency VCSELs and two dimensional (2D) VCSEL arrays for such illumination applications. GaAs-based VCSEL wafers are grown by MOCVD and processed into either top-emitting or bottom-emitting devices depending on the emission wavelength and applications. Results from both single devices and arrays are presented. In particular, record-high power conversion efficiency (PCE) of 63.4% with 300mW output was achieved from VCSELs at 1064nm. Such VCSELs also operate with <55% PCE at 50C. For a 2mm by 10mm array, 56.4% PCE with 150W output was demonstrated. Using those VCSELs and arrays as building blocks, various high power illuminators ranging from a few Watts to over 100 kiloWatts have been fabricated.
High-power red laser sources are used in many applications such as cosmetics, cancer photodynamic therapy, and DNA
sequencing in the medical field, laser-based RGB projection display, and bar-code scanning to name a few. Verticalcavity surface-emitting lasers (VCSELs) can be used as high-power laser sources, as efficient single devices can be configured into high-power two-dimensional arrays and scaled into modules of arrays. VCSELs emit in a circular,
uniform beam which can greatly reduce the complexity and cost of optics. Other advantages include a narrow and stable emission spectrum, low speckle of the far-field emission, and good reliability. However, developing efficient red
VCSEL sources presents some challenges because of the reduced quantum-well carrier confinement and the increased
Aluminum content (to avoid absorption) which increases thermal impedance, and also decreases the DBR index contrast resulting in increased penetration length and cavity losses. We have recently developed VCSEL devices lasing in the visible 6xx nm wavelength band, and reaching 30% power conversion efficiency. We fabricated high-power 2D arrays by removing the GaAs substrate entirely and soldered the chips on high thermal conductivity submounts. Such arrays have demonstrated several Watts of output power at room temperature, in continuous-wave (CW) operation. Several tens of Watts are obtained in QCW operation. Results and challenges of these high-power visible VCSEL arrays will be discussed.
Self-assembled superlattices (SASs) are intrinsically acentric and highly cross-linked structures. For organic electro-optics, they offer great advantages such as not requiring electric field poling for creating an acentric, EO-active microstructure and having excellent chemical, thermal, and orientational stabilities. In this paper, a greatly improved two-step all "wet-chemical" self-assembly (SA) approach is reported. Excellent radiation hardness of the SAS films is demonstrated by high-energy proton irradiation experiments. By introducing metal oxide nanolayers during SA, we show that the refractive indices of SAS films can be tuned over a wide range. Through special chromophore design, the optical absorption maxima of SAS films can also be greatly blue-shifted. Prototype waveguiding electro-optic modulators have been fabricated using the SAS films integrated with low-loss polymeric materials functioning as partial guiding and cladding layers. EO parameters such as the half-wave voltage and the effective electro-optic coefficient are reported.
Modal behavior of a 2-D (square lattice geometry) antiguided vertical cavity surface emitting laser (VCSEL) array was studied by 3-D bi-directional beam propagation method. Above threshold operation of leaky modes was simulated using multiple iterations. Besides, a method based on functions of Krylov’s subspace, was developed to find a number of array optical modes in a VCSEL array with gain and index distributions established by the oscillating mode. In calculations, both Fourier and space variable descriptions of beam propagation were combined. The FFT technique was used for calculations of the Fourier image and the original. Conditions are found for favorable lasing of the in-phase mode providing high laser beam quality. Experimentally realized 5x5 laser array was studied numerically.
The 2-D antiguided array results from shifting the cavity resonance between the element and inter-element regions and is fabricated by selective chemical etching and two-step metalorganic chemical vapor deposition (MOCVD) growth. In-phase and out-of-phase array mode operation is observed from top-emitting rectangular arrays as large as 400 elements, depending on the inter-element width, in good agreement with theory.
Cold-cavity modal behavior of a 2-D (4x4 square lattice geometry) antiguided vertical cavity surface emitting laser (VCSEL) array is studied by the means of an effective-index model and fiber-mode approximation. The calculations show that the 2-D array can operate under a resonant condition, provided that a resonance in both of the 1-D directions is satisfied. Although out-of-phase and adjacent modes will compete with the in-phase mode around its resonant position, our simulation shows that, with the introduction of inter-element loss, the in-phase mode can be favored to lase for a wide range of inter-element width, s, around its resonant position. The effective-index model is shown to be in qualitative agreement with a more comprehensive (exact) 3-D beam-propagation-based simulation, which takes into account the actual layered structure. The 2-D antiguides are constructed from shifting the cavity resonance between the element and inter-element regions and fabricated by chemically selective etching and two-step MOCVD growth. While both diffraction-limited resonant in-phase and out-of-phase modes are observed from top-emitting arrays, a 2-D bottom-emitting structure is adopted to improve heat removal. Preliminary results of 40 mW pulsed and 10 mW CW powers have been obtained from the junction up and down arrays respectively.
Novel single-mode VCSEL designs based on antiguided structures are discussed. Modal characteristics of single-element, 8-12m-dia, ARROW-type VCSELs and two-dimensional phase-locked arrays (4x4 elements) of antiguided VCSELs are studied. ARROW-type VCSELs exhibit low loss for the fundamental mode and strong high-order mode suppression as a result of edge radiation leakage. Preliminary results are 1mW single-mode (thermally limited) from 12im-dia devices. Two-dimensional arrays of antiguided VCSELs are shown to exhibit resonant coupling if the interelement spacing corresponds to an integer number of half-waves of the lateral radiation leakage. Single-mode emission with all array elements locked "in-phase" is demonstrated for the first time from a monolithic 2-D array of VCSELs. Large phase-locked arrays have potential for <100mW of single-mode output power with further optimization and efficient heatsinking.