Hans Opower

Power-scalable system of phase-locked single-mode diode lasers

The direct use of diode lasers for high-power applications in material processing is limited to applications with relatively low beam quality and power density requirements. To achieve high beam quality one must use single-mode diode lasers, however with the drawback of relatively low optical output powers from these components. To realize a high-power system while conserving the high beam quality of the individual emitters requires coherent coupling of the emitters. Such a power-scalable system consisting of 19 slave lasers that are injection locked by one master laser has been built and investigated, with low-power diode lasers used for system demonstration. The optical power of the 19 injection-locked lasers is coupled into polarization-maintaining single-mode fibers and geometrically superimposed by a lens array and a focusing lens. The phase of each emitter is controlled by a simple electronic phase-control loop. The coherence of each slave laser is stabilized by computer control of the laser current and guarantees a stable degree of coherence of the whole system of 0.7. An enhancement factor of 13.2 in peak power density compared with that which was achievable with the incoherent superposition of the diode lasers was observed.

High-Power Diode Lasers for Direct Applications

Diode-laser systems have a good chance to be used as light sources for direct applications. For this purpose a large number of relatively weak light bundles originating from individual oscillators have to be coupled to a single powerful beam. The coupling may be done in a completely incoherent way in cases where only a moderate beam quality is appropriate or with increasing degrees of coherence corresponding to applications where high demands on the quality are necessary. In this article all current methods of incoherent as well as of coherent beam combining are described and judged with respect to their present and future potential.

Autostable injection locking of a 4x4 VCSEL array with on-chip master laser

Stable phase-locking of a VCSEL array of N equals 16 emitters with the master laser on the same chip is demonstrated. To accomplish the injection-locking a very small portion of the master radiation is seeded frontally into the 16 slave lasers. All VCSELs are driven by one voltage source with small individual series resistors to compensate for frequency differences. The beams are collimated by a microlens array and are commonly focused. A factor 14 increase of the peak power density of the superimposed beams compared to the unlocked operation is achieved (overall system coherence 85%) without any phase control. The coherent operation is stable for hours without any sophisticated voltage or temperature control. By slightly varying the current of each emitter within the locking range a maximum phase shift of (pi) can be achieved for each emitter. In this way residual phase differences between the individual beams can be compensated. The fraction of the master power necessary to lock one slave laser is below 10-3. Therefore, scaling to very large arrays is possible. Apart from simply increasing the peak power density of the chip, a promising perspective for data transmission applications is the GHz-modulation of the system coherence. The locking of the array can be switched by a very small (2%) modulation of the master current thereby switching the peak power density by a factor of nearly N.

Coherent fiber coupling of laser diodes

Laser diodes with diffraction-limited beam quality offer high power densities of the order of 107 - 108 W/cm2, but are limited in output power to some watts. Scaling to higher powers has to be realized by superposition of a number of laser diodes. Coherent superposition allows us to further increase the power density in the far field. This is realized by injection locking of three slave laser diodes (Toshiba TOLD) 9140, 20 mW, 690 nm) by one master laser diode (TOLD 9140) and superpositioning of the three slaves lasers by a lens array. The feedback of the slaves into the master is suppressed by two Faraday isolators. For superpositioning the light of the slaves while maintaining the high beam quality, the light of each diode is coupled into an optical single-mode fiber. Phase shifts due to mechanical or thermal disturbances of the single-mode fibers for frequencies up to 1 kHz are compensated by a single-mode optical fiber piezoceramic phase modulator and an electronic control circuit. A phase stability with a maximum phase error smaller than 6 degrees is kept over an hour. The power-density distribution in the focal plane of the focusing lens shows a peak power 2.6 times that of the incoherent superposition and a modulation corresponding to the Fourier transform of the nearfield distribution of the lens array.

Systems of fiber-coupled diode lasers as versatile sources of high-brightness radiation

Incoherent superpositioning of radiation from a single-mode fiber bundle with 200 mW output power per fiber allows to realize power densities of 2 MW/cm2. The total power being directly scalable with the number of fibers. With special optics for imaging the fiber-bundle endface onto the target instead of simply focusing it is possible to control the power density within that spot. Coherent superpositioning allows to further increase the power density and to direct the beam to a spot within the field of the incoherent superposition. Such systems could be useful for all kinds of applications requiring high-brightness radiation like cutting and welding or laser projection, printing plates, lithography, etc.

Fiber coupling of single-mode laser diodes with power-density conservation

Laser diodes with diffraction limited beam quality offer high power densities of the order of iO - 108 W/cm2, but are limited in output power to some Watts. Scaling to higher powers without using a solidstate laser converter has to be realized by incoherent superposition of the outputs of a number of laser diodes. For that the radiation of single-mode laser diodes is coupled into single-mode fibers which at the other end are shaped into a bundle of hexagonal symmetry. The radiation leaving the fiber bundle is collimated with an array of achromats and focused with an additional lens onto the target. With 19 fibercoupled 690-nm diodes (TOLD 915 1, 20 mW at the fiber end) a total cw power of 338 mW in a spot of 19.4 jim diameter (at I/Is = l/e2of a nearly Gaussian cross section) was achieved. The peak power density was 263 kW/cm2, which is approximately 1 .7 times that of a single fiber. Optimizing the filling factor should further increase the power density. Keywords: single-mode lasers diodes, single-mode fibers, scalable system. incoherent coupling

High-power laser float-zone crystal growth

A float zone crystal growth apparatus was developed. The heat source was a 3.5 kW CO2 Trumpf laser with a long term stability of 2 %. The original laser beam was divided, in the beam delivery system, into three equivalent focusable beams. The vacuum vessel is equipped with two equivalent rotation - translation systems with a maximum translational speed of 1 mm/minute. The vessel is also pierced with five windows, three of which serve the three incoming beams, one for observation with a television camera, one for an optical pyrometer or for direct observation. The three beam windows are protected by a laminar gas curtain. A heat shield system with an after heater is planned, in order to enable growth of high temperature oxides. The system was tested in the growth of small sapphire crystals (0 4 mm) and in the growth of silicon whose dimensions were 0 10 mm x 70 mm. * On leave from the Weizmann Institute, Rehovot, Israel